TRẦN MINH TƯỜNG

Khởi động trang 15 Toán 9 Tập 1: Tại một cửa hàng, chị An mua 1,2 kg thịt lợn và 0,7 kg thịt bò hết 362 000 đồng; chị Ba mua 0,8 kg thịt lợn và 0,5 kg thịt bò cùng loại hết 250 000 đồng. Làm thế nào để tính được giá tiền 1 kg mỗi loại thịt lợn và thịt bò?<img alt="Khởi động trang 15 Toán 9 Tập 1 Chân trời sáng tạo | Giải Toán 9" src="https://vietjack.com/toan-9-ct/images/khoi-dong-trang-15-toan-9-tap-1.PNG" style="border: 0px; box-sizing: border-box; display: inline; margin-left: auto; margin-right: auto; max-width: 100%; padding-bottom: 4px; vertical-align: middle;" />Lời giải:Gọi x, y (đồng) lần lượt là giá tiền 1 kg mỗi loại thịt lợn và thịt bò (x, y > 0)Giá tiền 1,2 kg thịt lợn là 1,2x (đồng).Giá tiền 0,7 kg thịt bò là 0,7y (đồng).Số tiền chị An mua là hết 362 000 đồng nên ta có 1,2x + 0,7y = 362 000. (1)Giá tiền 0,8 kg thịt lợn là 0,8x (đồng).Giá tiền 0,5 kg thịt bò là 0,5y (đồng).Số tiền chị Ba mua là hết 250 000 đồng nên ta có 0,8x + 0,5y = 250 000.      (2)Từ (1) và (2) ta có hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>1,2</mn><mi>x</mi><mo>+</mo><mn>0,7</mn><mi>y</mi><mo>=</mo><mn>362</mn><mtext>&#xA0;</mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>0,8</mn><mi>x</mi><mo>+</mo><mn>0,5</mn><mi>y</mi><mo>=</mo><mn>250</mn><mtext>&#xA0;</mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{1,2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+0,7<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=362 0000,8<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+0,5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=250 000<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>1,2</mn><mi>�</mi><mo>+</mo><mn>0,7</mn><mi>�</mi><mo>=</mo><mn>362</mn><mtext> </mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>0,8</mn><mi>�</mi><mo>+</mo><mn>0,5</mn><mi>�</mi><mo>=</mo><mn>250</mn><mtext> </mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>Để tính được giá tiền 1 kg mỗi loại thịt lợn và thịt bò thì ta cần giải phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>1,2</mn><mi>x</mi><mo>+</mo><mn>0,7</mn><mi>y</mi><mo>=</mo><mn>362</mn><mtext>&#xA0;</mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>0,8</mn><mi>x</mi><mo>+</mo><mn>0,5</mn><mi>y</mi><mo>=</mo><mn>250</mn><mtext>&#xA0;</mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{1,2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+0,7<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=362 0000,8<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+0,5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=250 000<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>1,2</mn><mi>�</mi><mo>+</mo><mn>0,7</mn><mi>�</mi><mo>=</mo><mn>362</mn><mtext> </mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>0,8</mn><mi>�</mi><mo>+</mo><mn>0,5</mn><mi>�</mi><mo>=</mo><mn>250</mn><mtext> </mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>.1. Giải hệ phương trình bằng phương pháp thếThực hành 1 trang 16 Toán 9 Tập 1: Giải các hệ phương trình:a) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>+</mo><mn>2</mn><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>5</mn><mi>x</mi><mo>&#x2212;</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>11</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−25<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=11;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>=</mo><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>5</mn><mi>�</mi><mo>−</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>11</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>b) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>5</mn></mtd></mtr><mtr><mtd><mo>&#x2212;</mo><mn>2</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>11</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−5−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=11;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mo>−</mo><mn>5</mn></mtd></mtr><mtr><mtd><mo>−</mo><mn>2</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>11</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>c) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>6</mn><mi>x</mi><mo>+</mo><mn>2</mn><mi>y</mi><mo>=</mo><mn>4.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=26<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=4.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>6</mn><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>=</mo><mn>4.</mn></mtd></mtr></mtable></mfenced></math>Lời giải:a) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>+</mo><mn>2</mn><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>5</mn><mi>x</mi><mo>&#x2212;</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>11</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−25<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=11<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>=</mo><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>5</mn><mi>�</mi><mo>−</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>11</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mo>&#x2212;</mo><mn>2</mn><mi>y</mi><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>5</mn><mfenced><mrow><mo>&#x2212;</mo><mn>2</mn><mi>y</mi><mo>&#x2212;</mo><mn>2</mn></mrow></mfenced><mo>&#x2212;</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>11</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y−25(−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y−2)−4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=11<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mo>−</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>5</mn><mfenced><mrow><mo>−</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>2</mn></mrow></mfenced><mo>−</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>11</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mo>&#x2212;</mo><mn>2</mn><mi>y</mi><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mo>&#x2212;</mo><mn>10</mn><mi>y</mi><mo>&#x2212;</mo><mn>10</mn><mo>&#x2212;</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>11</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-6-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y−2−10<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y−10−4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=11<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mo>−</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mo>−</mo><mn>10</mn><mi>�</mi><mo>−</mo><mn>10</mn><mo>−</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>11</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mo>&#x2212;</mo><mn>2</mn><mi>y</mi><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>14</mn><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>21</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-7-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y−214<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−21<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mo>−</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>14</mn><mi>�</mi><mo>=</mo><mo>−</mo><mn>21</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-8-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=1<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−32<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></mtd></mtr></mtable></mfenced></math>Vậy hệ phương trình có nghiệm duy nhất là <mfenced><mrow><mn>1</mn><mo>;</mo><mtext>&#x2009;&#x2009;</mtext><mo>&#x2212;</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></mrow></mfenced></math>" id="MathJax-Element-9-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">(1;−32)<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced><mrow><mn>1</mn><mo>;</mo><mtext>  </mtext><mo>−</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></mrow></mfenced></math>.b) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>5</mn></mtd></mtr><mtr><mtd><mo>&#x2212;</mo><mn>2</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>11</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-10-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−5−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=11<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mo>−</mo><mn>5</mn></mtd></mtr><mtr><mtd><mo>−</mo><mn>2</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>11</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>+</mo><mn>5</mn></mtd></mtr><mtr><mtd><mo>&#x2212;</mo><mn>2</mn><mi>x</mi><mo>+</mo><mfenced><mrow><mn>2</mn><mi>x</mi><mo>+</mo><mn>5</mn></mrow></mfenced><mo>=</mo><mn>11</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-11-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+(2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5)=11<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>+</mo><mn>5</mn></mtd></mtr><mtr><mtd><mo>−</mo><mn>2</mn><mi>�</mi><mo>+</mo><mfenced><mrow><mn>2</mn><mi>�</mi><mo>+</mo><mn>5</mn></mrow></mfenced><mo>=</mo><mn>11</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>+</mo><mn>5</mn></mtd></mtr><mtr><mtd><mn>0</mn><mi>x</mi><mo>=</mo><mn>6</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-12-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+50<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=6<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>+</mo><mn>5</mn></mtd></mtr><mtr><mtd><mn>0</mn><mi>�</mi><mo>=</mo><mn>6</mn></mtd></mtr></mtable></mfenced></math>Phương trình 0x = 6 vô nghiệm.Vậy hệ phương trình vô nghiệm.c) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>6</mn><mi>x</mi><mo>+</mo><mn>2</mn><mi>y</mi><mo>=</mo><mn>4</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-13-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=26<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=4<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>6</mn><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>=</mo><mn>4</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mo>&#x2212;</mo><mn>3</mn><mi>x</mi></mtd></mtr><mtr><mtd><mn>6</mn><mi>x</mi><mo>+</mo><mn>2</mn><mfenced><mrow><mn>2</mn><mo>&#x2212;</mo><mn>3</mn><mi>x</mi></mrow></mfenced><mo>=</mo><mn>4</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-14-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x6<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2(2−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x)=4<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mo>−</mo><mn>3</mn><mi>�</mi></mtd></mtr><mtr><mtd><mn>6</mn><mi>�</mi><mo>+</mo><mn>2</mn><mfenced><mrow><mn>2</mn><mo>−</mo><mn>3</mn><mi>�</mi></mrow></mfenced><mo>=</mo><mn>4</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mo>&#x2212;</mo><mn>3</mn><mi>x</mi></mtd></mtr><mtr><mtd><mn>6</mn><mi>x</mi><mo>+</mo><mn>4</mn><mo>&#x2212;</mo><mn>6</mn><mi>x</mi><mo>=</mo><mn>4</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-15-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x6<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+4−6<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=4<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mo>−</mo><mn>3</mn><mi>�</mi></mtd></mtr><mtr><mtd><mn>6</mn><mi>�</mi><mo>+</mo><mn>4</mn><mo>−</mo><mn>6</mn><mi>�</mi><mo>=</mo><mn>4</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mo>&#x2212;</mo><mn>3</mn><mi>x</mi></mtd></mtr><mtr><mtd><mn>0</mn><mi>x</mi><mo>=</mo><mn>0</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-16-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x0<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=0<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mo>−</mo><mn>3</mn><mi>�</mi></mtd></mtr><mtr><mtd><mn>0</mn><mi>�</mi><mo>=</mo><mn>0</mn></mtd></mtr></mtable></mfenced></math>Phương trình 0x = 0 nghiệm đúng với mọi x ∈ ℝ.Vậy hệ phương trình có vô số nghiệm. Các nghiệm của hệ được viết như sau: <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>&#x2208;</mo><mi>&#x211D;</mi></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mo>&#x2212;</mo><mn>3</mn><mi>x</mi><mo>.</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-17-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x∈R<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x.2. Giải hệ phương trình bằng phương pháp cộng đại sốKhám phá 2 trang 17 Toán 9 Tập 1: Cho hai hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>=</mo><mn>6</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>5</mn><mo>;</mo></mtd></mtr></mtable></mfenced><mo>(</mo><mi>I</mi><mo>)</mo></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=6<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5;(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-right: 0.064em; padding-top: 0.461em; white-space: pre;">I)<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>=</mo><mn>6</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>5</mn><mo>;</mo></mtd></mtr></mtable></mfenced><mo>(</mo><mi>�</mi><mo>)</mo></math>       <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>5.</mn></mtd></mtr></mtable></mfenced><mo>(</mo><mi>I</mi><mi>I</mi><mo>)</mo></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=1<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5.(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-right: 0.064em; padding-top: 0.461em; white-space: pre;">I<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-right: 0.064em; padding-top: 0.461em; white-space: pre;">I)<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>5.</mn></mtd></mtr></mtable></mfenced><mo>(</mo><mi>�</mi><mi>�</mi><mo>)</mo></math>a) Giải hệ phương trình (I) và hệ phương trình (II) bằng phương pháp thế. Có nhận xét gì về nghiệm của hai hệ này?b) Bằng cách cộng từng vế hai phương trình của hệ (II), ta nhận được một phương trình mới. Thay phương trình thứ nhất của hệ (II) bằng phương trình mới đó. Có nhận xét gì về kết quả nhận được?Lời giải:a) Xét hệ (I): <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>=</mo><mn>6</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=6<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>=</mo><mn>6</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>2</mn><mo>+</mo><mi>y</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=22+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>2</mn><mo>+</mo><mi>�</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=3<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math>Do đó, hệ phương trình (I) có nghiệm duy nhất là (2; 3).Xét hệ (II): <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-6-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=1<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>+</mo><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>1</mn><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-7-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−1<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−1=5<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>1</mn><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>1</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>=</mo><mn>6</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-8-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−13<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=6<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>1</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>=</mo><mn>6</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>=</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-9-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−1<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>=</mo><mn>2.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-10-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=2.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2.</mn></mtd></mtr></mtable></mfenced></math>Do đó, hệ phương trình (II) có nghiệm duy nhất là (2; 3).Nhận xét: Hệ phương trình (I) và hệ phương trình (II) có nghiệm như nhau.b) Cộng từng vế hai phương trình của hệ (II), ta nhận được một phương trình 3x = 6.Thay phương trình thứ nhất của hệ (II) bằng phương trình 3x = 6, ta có hệ <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>=</mo><mn>6</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced><mo>.</mo></math>" id="MathJax-Element-11-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=6<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>=</mo><mn>6</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced><mo>.</mo></math>Nhận xét: Ta thu được hệ phương trình giống với hệ phương trình (I).Thực hành 2 trang 18 Toán 9 Tập 1: Giải các hệ phương trình:a) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>5</mn><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>14</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>x</mi><mo>+</mo><mn>3</mn><mi>y</mi><mo>=</mo><mn>2</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−142<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mn>5</mn><mi>�</mi><mo>=</mo><mo>−</mo><mn>14</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>+</mo><mn>3</mn><mi>�</mi><mo>=</mo><mn>2</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>b) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>y</mi><mo>=</mo><mn>15</mn></mtd></mtr><mtr><mtd><mn>6</mn><mi>x</mi><mo>&#x2212;</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>11.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=156<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=11.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>=</mo><mn>15</mn></mtd></mtr><mtr><mtd><mn>6</mn><mi>�</mi><mo>−</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>11.</mn></mtd></mtr></mtable></mfenced></math>Lời giải:a) Trừ từng vế hai phương trình của hệ đã cho, ta được –8y = –16. Suy ra y = 2.Thay y = 2 vào phương trình thứ hai của hệ, ta được 2x + 3 . 2 = 2 hay 2x = –4. Do đó x = –2.Vậy hệ phương trình có nghiệm duy nhất (–2; 2).b) Nhân hai vế của phương trình thứ nhất với 3 và nhân hai vế của phương trình thứ hai với 2, ta được:<mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>12</mn><mi>x</mi><mo>+</mo><mn>15</mn><mi>y</mi><mo>=</mo><mn>45</mn></mtd></mtr><mtr><mtd><mn>12</mn><mi>x</mi><mo>&#x2212;</mo><mn>8</mn><mi>y</mi><mo>=</mo><mn>22.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+15<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=4512<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−8<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=22.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>12</mn><mi>�</mi><mo>+</mo><mn>15</mn><mi>�</mi><mo>=</mo><mn>45</mn></mtd></mtr><mtr><mtd><mn>12</mn><mi>�</mi><mo>−</mo><mn>8</mn><mi>�</mi><mo>=</mo><mn>22.</mn></mtd></mtr></mtable></mfenced></math>Trừ từng vế hai phương trình của hệ, ta được 23y = 23. Suy ra y = 1.Thay y = 1 vào phương trình thứ nhất của hệ đã cho, ta được 4x + 5 . 1 = 15 hay 4x = 10. Do đó <mi>x</mi><mo>=</mo><mfrac><mn>5</mn><mn>2</mn></mfrac><mo>.</mo></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=52.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mfrac><mn>5</mn><mn>2</mn></mfrac><mo>.</mo></math>Vậy hệ phương trình có nghiệm duy nhất <mfenced><mrow><mfrac><mn>5</mn><mn>2</mn></mfrac><mtext>&#x2009;</mtext><mo>;</mo><mtext>&#x2009;&#x2009;</mtext><mn>1</mn></mrow></mfenced><mo>.</mo></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">(52;1). Vận dụng 1 trang 18 Toán 9 Tập 1: Xác định a, b để đồ thị hàm số y = ax + b đi qua hai điểm A(2; –2) và B(–1; 3).Lời giải:• Đồ thị hàm số y = ax + b đi qua điểm A(2; –2) nên–2 = a . 2 + b hay 2a + b = –2. (1)• Đồ thị hàm số y = ax + b đi qua điểm B(–1; 3) nên3 = a . (–1) + b hay –a + b = 3.  (2)Từ (1) và (2) ta có hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>a</mi><mo>+</mo><mi>b</mi><mo>=</mo><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mo>&#x2212;</mo><mi>a</mi><mo>+</mo><mi>b</mi><mo>=</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">b=−2−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">b=3<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mo>−</mo><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math>(I)Trừ từng vế hai phương trình của hệ (I), ta được 3a = –5. Suy ra <mi>a</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a=−53<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></math>Thay <mi>a</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a=−53<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></math> vào phương trình thứ hai của hệ (I), ta được <mfrac><mn>5</mn><mn>3</mn></mfrac><mo>+</mo><mi>b</mi><mo>=</mo><mn>3</mn></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">53+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">b=3<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>5</mn><mn>3</mn></mfrac><mo>+</mo><mi>�</mi><mo>=</mo><mn>3</mn></math>. Do đó <mi>b</mi><mo>=</mo><mfrac><mn>4</mn><mn>3</mn></mfrac><mo>.</mo></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">b=43.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mfrac><mn>4</mn><mn>3</mn></mfrac><mo>.</mo></math>Vậy <mi>a</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>5</mn><mn>3</mn></mfrac><mo>;</mo><mo>&#xA0;</mo><mi>b</mi><mo>=</mo><mfrac><mn>4</mn><mn>3</mn></mfrac><mo>.</mo></math>" id="MathJax-Element-6-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a=−53; <span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">b=43.3. Tìm nghiệm của hệ hai phương trình bậc nhất hai ẩn bằng máy tính cầm tayThực hành 3 trang 19 Toán 9 Tập 1: Tìm nghiệm của các hệ phương trình sau bằng máy tính cầm tay:a) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>19</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=43<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−19;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>=</mo><mo>−</mo><mn>19</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>b) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mo>&#x2212;</mo><mn>3</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>y</mi><mo>=</mo><mn>12</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>5.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=122<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mo>−</mo><mn>3</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>=</mo><mn>12</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>5.</mn></mtd></mtr></mtable></mfenced></math>Lời giải:a) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>19</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=43<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−19;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>=</mo><mo>−</mo><mn>19</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>– Ấn nút ON để khởi động máy.– Ấn nút MODE, ấn nút 5, ấn nút 1, rồi nhập các hệ số như sau:<img alt="Thực hành 3 trang 19 Toán 9 Tập 1 Chân trời sáng tạo | Giải Toán 9" src="https://vietjack.com/toan-9-ct/images/thuc-hanh-3-trang-19-toan-9-tap-1.PNG" style="border: 0px; box-sizing: border-box; display: inline; margin-left: auto; margin-right: auto; max-width: 100%; padding-bottom: 4px; vertical-align: middle;" />Màn hình hiện ra kết quả như hình sau:<img alt="Thực hành 3 trang 19 Toán 9 Tập 1 Chân trời sáng tạo | Giải Toán 9" src="https://vietjack.com/toan-9-ct/images/thuc-hanh-3-trang-19-toan-9-tap-1a.PNG" style="border: 0px; box-sizing: border-box; display: inline; margin-left: auto; margin-right: auto; max-width: 100%; padding-bottom: 4px; vertical-align: middle;" />– Ấn <img alt="Thực hành 3 trang 19 Toán 9 Tập 1 Chân trời sáng tạo | Giải Toán 9" src="https://vietjack.com/toan-9-ct/images/thuc-hanh-3-trang-19-toan-9-tap-1b.PNG" style="border: 0px; box-sizing: border-box; display: inline; margin-left: auto; margin-right: auto; max-width: 100%; padding-bottom: 4px; vertical-align: middle;" />, kết quả như hình sau:<img alt="Thực hành 3 trang 19 Toán 9 Tập 1 Chân trời sáng tạo | Giải Toán 9" src="https://vietjack.com/toan-9-ct/images/thuc-hanh-3-trang-19-toan-9-tap-1c.PNG" style="border: 0px; box-sizing: border-box; display: inline; margin-left: auto; margin-right: auto; max-width: 100%; padding-bottom: 4px; vertical-align: middle;" />Vậy hệ phương trình có nghiệm duy nhất là <mfenced><mrow><mfrac><mn>1</mn><mn>13</mn></mfrac><mtext>&#x2009;</mtext><mo>;</mo><mtext>&#x2009;&#x2009;</mtext><mo>&#x2212;</mo><mfrac><mn>50</mn><mn>13</mn></mfrac></mrow></mfenced><mo>.</mo></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">(113;−5013).<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced><mrow><mfrac><mn>1</mn><mn>13</mn></mfrac><mtext> </mtext><mo>;</mo><mtext>  </mtext><mo>−</mo><mfrac><mn>50</mn><mn>13</mn></mfrac></mrow></mfenced><mo>.</mo></math>b) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mo>&#x2212;</mo><mn>3</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>y</mi><mo>=</mo><mn>12</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>5.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=122<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mo>−</mo><mn>3</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>=</mo><mn>12</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>5.</mn></mtd></mtr></mtable></mfenced></math>– Ấn nút ON để khởi động máy.– Ấn nút MODE, ấn nút 5, ấn nút 1, rồi nhập các hệ số như sau:<img alt="Thực hành 3 trang 19 Toán 9 Tập 1 Chân trời sáng tạo | Giải Toán 9" src="https://vietjack.com/toan-9-ct/images/thuc-hanh-3-trang-19-toan-9-tap-1-a.PNG" style="border: 0px; box-sizing: border-box; display: inline; margin-left: auto; margin-right: auto; max-width: 100%; padding-bottom: 4px; vertical-align: middle;" />Màn hình hiện ra kết quả như hình sau:<img alt="Thực hành 3 trang 19 Toán 9 Tập 1 Chân trời sáng tạo | Giải Toán 9" src="https://vietjack.com/toan-9-ct/images/thuc-hanh-3-trang-19-toan-9-tap-1-1a.PNG" style="border: 0px; box-sizing: border-box; display: inline; margin-left: auto; margin-right: auto; max-width: 100%; padding-bottom: 4px; vertical-align: middle;" />– Ấn <img alt="Thực hành 3 trang 19 Toán 9 Tập 1 Chân trời sáng tạo | Giải Toán 9" src="https://vietjack.com/toan-9-ct/images/thuc-hanh-3-trang-19-toan-9-tap-1b.PNG" style="border: 0px; box-sizing: border-box; display: inline; margin-left: auto; margin-right: auto; max-width: 100%; padding-bottom: 4px; vertical-align: middle;" />, kết quả như hình sau:<img alt="Thực hành 3 trang 19 Toán 9 Tập 1 Chân trời sáng tạo | Giải Toán 9" src="https://vietjack.com/toan-9-ct/images/thuc-hanh-3-trang-19-toan-9-tap-1-2a.PNG" style="border: 0px; box-sizing: border-box; display: inline; margin-left: auto; margin-right: auto; max-width: 100%; padding-bottom: 4px; vertical-align: middle;" />Vậy hệ phương trình có nghiệm duy nhất là (1; 3).4. Giải bài toán bằng cách lập hệ phương trìnhKhám phá 3 trang 19 Toán 9 Tập 1: Hai lớp 9A và 9B có tổng số 82 học sinh. Trong dịp tết trồng cây năm 2022, mỗi học sinh lớp 9A trồng được 3 cây, mỗi học sinh lớp 9B trồng được 4 cây nên cả hai lớp trồng được tổng số 288 cây. Gọi x, y lần lượt là số học sinh lớp 9A và lớp 9B (x ∈ ℕ*, y ∈ ℕ*).a) Từ dữ liệu đã cho, lập hai phương trình bậc nhất hai ẩn biểu thị số học sinh của hai lớp và số cây trồng được.b) Giải hệ hai phương trình bậc nhất hai ẩn và cho biết mỗi lớp bao nhiêu học sinh.Lời giải:a) Hai lớp 9A và 9B có tổng số 82 học sinh nên x + y = 82.   (1)Số cây lớp 9A trồng được là: 3x (cây).Số cây lớp 9B trồng được là: 4y (cây).Theo đề bài, ta có phương trình 3x + 4y = 288.      (2)Từ (1) và (2) ta có hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>82</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>288</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=823<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=288<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>82</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>288</mn></mtd></mtr></mtable></mfenced></math>b) Từ hệ phương trình ở câu a, ta có <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>82</mn><mo>&#x2212;</mo><mi>x</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>288</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=82−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=288<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>82</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>288</mn></mtd></mtr></mtable></mfenced></math>                                                        <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>82</mn><mo>&#x2212;</mo><mi>x</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>4</mn><mfenced><mrow><mn>82</mn><mo>&#x2212;</mo><mi>x</mi></mrow></mfenced><mo>=</mo><mn>288</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=82−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+4(82−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x)=288<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>82</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>4</mn><mfenced><mrow><mn>82</mn><mo>−</mo><mi>�</mi></mrow></mfenced><mo>=</mo><mn>288</mn></mtd></mtr></mtable></mfenced></math>                                                        <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>82</mn><mo>&#x2212;</mo><mi>x</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>328</mn><mo>&#x2212;</mo><mn>4</mn><mi>x</mi><mo>=</mo><mn>288</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=82−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+328−4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=288<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>82</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>328</mn><mo>−</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>288</mn></mtd></mtr></mtable></mfenced></math>                                                        <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>82</mn><mo>&#x2212;</mo><mi>x</mi></mtd></mtr><mtr><mtd><mi>x</mi><mo>=</mo><mn>40</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=82−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=40<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>82</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>40</mn></mtd></mtr></mtable></mfenced></math>                                                        <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>42</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>=</mo><mn>40</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-6-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=42<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=40<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>42</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>40</mn></mtd></mtr></mtable></mfenced></math> (thỏa mãn)Vậy lớp 9A có 40 học sinh, lớp 9B có 42 học sinh.Thực hành 4 trang 20 Toán 9 Tập 1: Một mảnh vườn hình chữ nhật có chu vi 64 m. Nếu tăng chiều dài thêm 2 m và tăng chiều rộng thêm 3 m thì diện tích tăng thêm 88 m2. Tính chiều dài, chiều rộng của mảnh vườn đó.Lời giải:Gọi x (m), y (m) lần lượt là chiều dài và chiều rộng ban đầu của mảnh vườn hình chữ nhật (0 < y < x < 64).Diện tích ban đầu của mảnh vườn hình chữ nhật đó là: xy (m2).Chu vi mảnh vườn hình chữ nhật là 64 m nên 2(x + y) = 64 hay x + y = 32.       (1)Chiều dài mảnh vườn hình chữ nhật sau khi tăng là: x + 2 (m)Chiều rộng mảnh vườn hình chữ nhật sau khi tăng là: y + 3 (m)Diện tích mảnh vườn hình chữ nhật sau khi tăng kích thước là: (x + 2)(y + 3) (m2).Theo bài, sau khi tăng chiều dài và chiều rộng thì diện tích mảnh vườn tăng thêm 88 m2 nên ta có phương trình:(x + 2)(y + 3) = xy + 88xy + 3x + 2y + 6 = xy + 883x + 2y = 82.         (2)Từ (1) và (2) ta có hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>32</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>2</mn><mi>y</mi><mo>=</mo><mn>82</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=323<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=82<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>32</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>=</mo><mn>82</mn></mtd></mtr></mtable></mfenced></math>                                                         <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>32</mn><mo>&#x2212;</mo><mi>x</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>2</mn><mi>y</mi><mo>=</mo><mn>82</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=32−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=82<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>32</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>=</mo><mn>82</mn></mtd></mtr></mtable></mfenced></math>                                                         <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>32</mn><mo>&#x2212;</mo><mi>x</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>2</mn><mfenced><mrow><mn>32</mn><mo>&#x2212;</mo><mi>x</mi></mrow></mfenced><mo>=</mo><mn>82</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=32−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2(32−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x)=82<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>32</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>2</mn><mfenced><mrow><mn>32</mn><mo>−</mo><mi>�</mi></mrow></mfenced><mo>=</mo><mn>82</mn></mtd></mtr></mtable></mfenced></math>                                                         <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>32</mn><mo>&#x2212;</mo><mi>x</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>64</mn><mo>&#x2212;</mo><mn>2</mn><mi>x</mi><mo>=</mo><mn>82</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=32−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+64−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=82<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>32</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>64</mn><mo>−</mo><mn>2</mn><mi>�</mi><mo>=</mo><mn>82</mn></mtd></mtr></mtable></mfenced></math>                                                         <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>32</mn><mo>&#x2212;</mo><mi>x</mi></mtd></mtr><mtr><mtd><mi>x</mi><mo>=</mo><mn>18</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=32−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=18<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>32</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>18</mn></mtd></mtr></mtable></mfenced></math>                                                         <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mn>14</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>=</mo><mn>18</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-6-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=14<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=18<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>14</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>18</mn></mtd></mtr></mtable></mfenced></math> (thỏa mãn).Vậy chiều dài của mảnh vườn là 18 m, chiều rộng của mảnh vườn là 14 m.Thực hành 5 trang 20 Toán 9 Tập 1: Cân bằng phương trình hóa học sau bằng phương pháp đại số.NO + O2 → NO2Lời giải:Gọi x, y lần lượt là hệ số của NO và O2 thỏa mãn cân bằng phương trình hóa họcxNO + yO2 → NO2Cân bằng số nguyên tử N, số nguyên tử O ở hai vế, ta được <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>y</mi><mo>=</mo><mn>1.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=1.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>=</mo><mn>1.</mn></mtd></mtr></mtable></mfenced></math>Giải hệ phương trình này, ta được x = 1, y = <mfrac><mn>1</mn><mn>2</mn></mfrac></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">12<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>1</mn><mn>2</mn></mfrac></math>Đưa các hệ số tìm được vào phương trình hóa học, ta có<mi>N</mi><mi>O</mi><mo>+</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><msub><mi>O</mi><mn>2</mn></msub><mo>&#x2192;</mo><mi>N</mi><msub><mi>O</mi><mn>2</mn></msub></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-right: 0.085em; padding-top: 0.461em; white-space: pre;">N<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">O+12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">O2→<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-right: 0.085em; padding-top: 0.461em; white-space: pre;">N<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">O2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mi>�</mi><mo>+</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><msub><mi>�</mi><mn>2</mn></msub><mo>→</mo><mi>�</mi><msub><mi>�</mi><mn>2</mn></msub></math>Do các hệ số của phương trình hóa học phải là số nguyên nên nhân hai vế của phương trình hóa học với 2, ta được2NO + O2 → 2NO2Vận dụng 1 trang 20 Toán 9 Tập 1: Giải bài toán trong Hoạt động khởi động (trang 15).Lời giải:Gọi x, y (đồng) lần lượt là giá tiền 1 kg mỗi loại thịt lợn và thịt bò (x, y > 0)Giá tiền 1,2 kg thịt lợn là 1,2x (đồng).Giá tiền 0,7 kg thịt bò là 0,7y (đồng).Số tiền chị An mua là hết 362 000 đồng nên ta có 1,2x + 0,7y = 362 000. (1)Giá tiền 0,8 kg thịt lợn là 0,8x (đồng).Giá tiền 0,5 kg thịt bò là 0,5y (đồng).Số tiền chị Ba mua là hết 250 000 đồng nên ta có 0,8x + 0,5y = 250 000.      (2)Từ (1) và (2) ta có hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>1,2</mn><mi>x</mi><mo>+</mo><mn>0,7</mn><mi>y</mi><mo>=</mo><mn>362</mn><mtext>&#xA0;</mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>0,8</mn><mi>x</mi><mo>+</mo><mn>0,5</mn><mi>y</mi><mo>=</mo><mn>250</mn><mtext>&#xA0;</mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{1,2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+0,7<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=362 0000,8<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+0,5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=250 000<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>1,2</mn><mi>�</mi><mo>+</mo><mn>0,7</mn><mi>�</mi><mo>=</mo><mn>362</mn><mtext> </mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>0,8</mn><mi>�</mi><mo>+</mo><mn>0,5</mn><mi>�</mi><mo>=</mo><mn>250</mn><mtext> </mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>.Để tính được giá tiền 1 kg mỗi loại thịt lợn và thịt bò thì ta cần giải phương trình thu được.Ta giải phương trình:<mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>1,2</mn><mi>x</mi><mo>+</mo><mn>0,7</mn><mi>y</mi><mo>=</mo><mn>362</mn><mtext>&#xA0;</mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>0,8</mn><mi>x</mi><mo>+</mo><mn>0,5</mn><mi>y</mi><mo>=</mo><mn>250</mn><mtext>&#xA0;</mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{1,2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+0,7<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=362 0000,8<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+0,5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=250 000<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>1,2</mn><mi>�</mi><mo>+</mo><mn>0,7</mn><mi>�</mi><mo>=</mo><mn>362</mn><mtext> </mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>0,8</mn><mi>�</mi><mo>+</mo><mn>0,5</mn><mi>�</mi><mo>=</mo><mn>250</mn><mtext> </mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>12</mn><mi>x</mi><mo>+</mo><mn>7</mn><mi>y</mi><mo>=</mo><mn>3</mn><mtext>&#x2009;&#x2009;</mtext><mn>620</mn><mtext>&#x2009;&#x2009;</mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>8</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>y</mi><mo>=</mo><mn>2</mn><mtext>&#x2009;&#x2009;</mtext><mn>500</mn><mtext>&#x2009;&#x2009;</mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+7<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=36200008<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2500000<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>12</mn><mi>�</mi><mo>+</mo><mn>7</mn><mi>�</mi><mo>=</mo><mn>3</mn><mtext>  </mtext><mn>620</mn><mtext>  </mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>8</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>=</mo><mn>2</mn><mtext>  </mtext><mn>500</mn><mtext>  </mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>Nhân hai vế của phương trình thứ nhất với 4, nhân hai vế của phương trình thứ hai với 6, ta được:<mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>48</mn><mi>x</mi><mo>+</mo><mn>28</mn><mi>y</mi><mo>=</mo><mn>14</mn><mtext>&#x2009;&#x2009;</mtext><mn>480</mn><mtext>&#x2009;&#x2009;</mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>48</mn><mi>x</mi><mo>+</mo><mn>30</mn><mi>y</mi><mo>=</mo><mn>15</mn><mtext>&#x2009;&#x2009;</mtext><mn>000</mn><mtext>&#x2009;&#x2009;</mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{48<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+28<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=1448000048<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+30<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=15000000<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>48</mn><mi>�</mi><mo>+</mo><mn>28</mn><mi>�</mi><mo>=</mo><mn>14</mn><mtext>  </mtext><mn>480</mn><mtext>  </mtext><mn>000</mn></mtd></mtr><mtr><mtd><mn>48</mn><mi>�</mi><mo>+</mo><mn>30</mn><mi>�</mi><mo>=</mo><mn>15</mn><mtext>  </mtext><mn>000</mn><mtext>  </mtext><mn>000</mn></mtd></mtr></mtable></mfenced></math>Trừ từng vế của phương trình thứ nhất cho phương trình thứ hai của hệ trên, ta được:–2y = – 520 000, suy ra y = 260 000 (thỏa mãn).Thay y = 260 000 vào phương trình 8x + 5y = 2 500 000, ta được:8x + 5 . 260 000 = 2 500 0008x = 1 200 000x = 150 000 (thỏa mãn).Vậy giá tiền 1 kg thịt lợn là 150 000 đồng và giá tiền 1 kg thịt bò là 260 000 đồng.Bài tậpBài 1 trang 21 Toán 9 Tập 1: Giải các hệ phương trình:a) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>7</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=32<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=7;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>7</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>b) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>&#x2212;</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>2</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=33<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>−</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>2</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>c) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>8</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−22<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−8;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>=</mo><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mo>−</mo><mn>8</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>d) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mo>&#x2212;</mo><mn>3</mn><mi>y</mi><mo>=</mo><mn>5.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=3−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mo>−</mo><mn>3</mn><mi>�</mi><mo>=</mo><mn>5.</mn></mtd></mtr></mtable></mfenced></math>Lời giải:a) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=32<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=7<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-6-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−7<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>7</mn><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-7-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−7=3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−7<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>7</mn><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>5</mn><mi>x</mi><mo>=</mo><mn>10</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-8-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=10<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−7<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>5</mn><mi>�</mi><mo>=</mo><mn>10</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-9-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−7<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-10-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−3<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mo>−</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math>Vậy hệ phương trình có nghiệm duy nhất là (2; −3).b) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>&#x2212;</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-11-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=33<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>−</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mi>y</mi><mo>+</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>3</mn><mfenced><mrow><mi>y</mi><mo>+</mo><mn>3</mn></mrow></mfenced><mo>&#x2212;</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-12-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y+33(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y+3)−4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mi>�</mi><mo>+</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>3</mn><mfenced><mrow><mi>�</mi><mo>+</mo><mn>3</mn></mrow></mfenced><mo>−</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mi>y</mi><mo>+</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>y</mi><mo>+</mo><mn>9</mn><mo>&#x2212;</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-13-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y+33<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y+9−4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mi>�</mi><mo>+</mo><mn>3</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>9</mn><mo>−</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mi>y</mi><mo>+</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-14-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y+3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=7<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mi>�</mi><mo>+</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>10</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-15-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=10<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=7<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>10</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>7</mn></mtd></mtr></mtable></mfenced></math>Vậy hệ phương trình có nghiệm duy nhất là (10; 7).c) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-16-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−22<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−8<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>=</mo><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mo>−</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>+</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-17-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+8<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>=</mo><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>+</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>x</mi><mo>+</mo><mn>5</mn><mfenced><mrow><mn>2</mn><mi>x</mi><mo>+</mo><mn>8</mn></mrow></mfenced><mo>=</mo><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>+</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-18-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5(2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+8)=−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+8<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>�</mi><mo>+</mo><mn>5</mn><mfenced><mrow><mn>2</mn><mi>�</mi><mo>+</mo><mn>8</mn></mrow></mfenced><mo>=</mo><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>+</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>x</mi><mo>+</mo><mn>10</mn><mi>x</mi><mo>+</mo><mn>40</mn><mo>=</mo><mo>&#x2212;</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>+</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-19-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+10<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+40=−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+8<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>�</mi><mo>+</mo><mn>10</mn><mi>�</mi><mo>+</mo><mn>40</mn><mo>=</mo><mo>−</mo><mn>2</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>+</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>14</mn><mi>x</mi><mo>=</mo><mo>&#x2212;</mo><mn>42</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>+</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-20-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{14<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=−42<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+8<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>14</mn><mi>�</mi><mo>=</mo><mo>−</mo><mn>42</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn><mi>�</mi><mo>+</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mo>&#x2212;</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-21-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mo>−</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math>Vậy hệ phương trình có nghiệm duy nhất là (−3; 2).d) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mo>&#x2212;</mo><mn>3</mn><mi>y</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-22-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=3−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mo>−</mo><mn>3</mn><mi>�</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>&#x2212;</mo><mfrac><mn>5</mn><mn>3</mn></mfrac><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-23-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−53=3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−53<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>−</mo><mfrac><mn>5</mn><mn>3</mn></mfrac><mo>=</mo><mn>3</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>=</mo><mfrac><mn>14</mn><mn>3</mn></mfrac></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-24-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=143<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−53<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>=</mo><mfrac><mn>14</mn><mn>3</mn></mfrac></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mfrac><mn>14</mn><mn>9</mn></mfrac></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-25-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=149<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−53<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mfrac><mn>14</mn><mn>9</mn></mfrac></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></mtd></mtr></mtable></mfenced></math>Vậy hệ phương trình có nghiệm duy nhất là <mfenced><mrow><mfrac><mn>14</mn><mn>9</mn></mfrac><mo>;</mo><mtext>&#x2009;&#x2009;</mtext><mo>&#x2212;</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></mrow></mfenced></math>" id="MathJax-Element-26-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">(149;−53)Bài 2 trang 21 Toán 9 Tập 1: Giải các hệ phương trình:a) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mfrac><mn>4</mn><mn>3</mn></mfrac><mi>x</mi><mo>+</mo><mfrac><mn>1</mn><mn>3</mn></mfrac><mi>y</mi><mo>=</mo><mn>1</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=243<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+13<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=1;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mfrac><mn>4</mn><mn>3</mn></mfrac><mi>�</mi><mo>+</mo><mfrac><mn>1</mn><mn>3</mn></mfrac><mi>�</mi><mo>=</mo><mn>1</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>b) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>0</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>x</mi><mo>+</mo><mi>y</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>3</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y√2=02<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y√2=3;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>−</mo><mi>�</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>0</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>+</mo><mi>�</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>3</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>c) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>5</mn><mi>x</mi><msqrt><mn>3</mn></msqrt><mo>+</mo><mi>y</mi><mo>=</mo><mn>2</mn><msqrt><mn>2</mn></msqrt></mtd></mtr><mtr><mtd><mi>x</mi><msqrt><mn>6</mn></msqrt><mo>&#x2212;</mo><mi>y</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>2</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x√3+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2√2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x√6−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y√2=2;<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>5</mn><mi>�</mi><msqrt><mn>3</mn></msqrt><mo>+</mo><mi>�</mi><mo>=</mo><mn>2</mn><msqrt><mn>2</mn></msqrt></mtd></mtr><mtr><mtd><mi>�</mi><msqrt><mn>6</mn></msqrt><mo>−</mo><mi>�</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>2</mn><mo>;</mo></mtd></mtr></mtable></mfenced></math>d) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mfenced><mrow><mi>x</mi><mo>+</mo><mi>y</mi></mrow></mfenced><mo>+</mo><mn>3</mn><mfenced><mrow><mi>x</mi><mo>&#x2212;</mo><mi>y</mi></mrow></mfenced><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mfenced><mrow><mi>x</mi><mo>+</mo><mi>y</mi></mrow></mfenced><mo>+</mo><mn>2</mn><mfenced><mrow><mi>x</mi><mo>&#x2212;</mo><mi>y</mi></mrow></mfenced><mo>=</mo><mn>5.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y)+3(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y)=4(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y)+2(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y)=5.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mfenced><mrow><mi>�</mi><mo>+</mo><mi>�</mi></mrow></mfenced><mo>+</mo><mn>3</mn><mfenced><mrow><mi>�</mi><mo>−</mo><mi>�</mi></mrow></mfenced><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mfenced><mrow><mi>�</mi><mo>+</mo><mi>�</mi></mrow></mfenced><mo>+</mo><mn>2</mn><mfenced><mrow><mi>�</mi><mo>−</mo><mi>�</mi></mrow></mfenced><mo>=</mo><mn>5.</mn></mtd></mtr></mtable></mfenced></math>Lời giải:a) Nhân hai vế của phương trình thứ hai với 3, ta được<mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>4</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=24<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=3<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>4</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>4</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math>Trừ từng vế hai phương trình của hệ mới, ta được 0x = −1. Phương trình này vô nghiệm.Vậy hệ phương trình đã cho vô nghiệm.b) Cộng từng vế hai phương trình của hệ đã cho, ta được 3x = 3. Suy ra x = 1.Thay x = 1 vào phương trình thứ nhất của hệ đã cho, ta được <mn>1</mn><mo>&#x2212;</mo><mi>y</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>0</mn></math>" id="MathJax-Element-6-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">1−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y√2=0<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mn>1</mn><mo>−</mo><mi>�</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>0</mn></math>. Do đó <mi>y</mi><mo>=</mo><mfrac><msqrt><mn>2</mn></msqrt><mn>2</mn></mfrac><mo>.</mo></math>" id="MathJax-Element-7-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=√22.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mfrac><msqrt><mn>2</mn></msqrt><mn>2</mn></mfrac><mo>.</mo></math>Vậy hệ phương trình có nghiệm duy nhất <mfenced><mrow><mn>1</mn><mtext>&#x2009;</mtext><mo>;</mo><mtext>&#x2009;&#x2009;</mtext><mfrac><msqrt><mn>2</mn></msqrt><mn>2</mn></mfrac></mrow></mfenced><mo>.</mo></math>" id="MathJax-Element-8-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">(1;√22).<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced><mrow><mn>1</mn><mtext> </mtext><mo>;</mo><mtext>  </mtext><mfrac><msqrt><mn>2</mn></msqrt><mn>2</mn></mfrac></mrow></mfenced><mo>.</mo></math>c) Chia hai vế của phương trình thứ hai cho <msqrt><mn>2</mn></msqrt></math>" id="MathJax-Element-9-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">√2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>2</mn></msqrt></math>, ta được<mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>5</mn><mi>x</mi><msqrt><mn>3</mn></msqrt><mo>+</mo><mi>y</mi><mo>=</mo><mn>2</mn><msqrt><mn>2</mn></msqrt></mtd></mtr><mtr><mtd><mi>x</mi><msqrt><mn>3</mn></msqrt><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><msqrt><mn>2</mn></msqrt></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-10-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x√3+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=2√2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x√3−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=√2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>5</mn><mi>�</mi><msqrt><mn>3</mn></msqrt><mo>+</mo><mi>�</mi><mo>=</mo><mn>2</mn><msqrt><mn>2</mn></msqrt></mtd></mtr><mtr><mtd><mi>�</mi><msqrt><mn>3</mn></msqrt><mo>−</mo><mi>�</mi><mo>=</mo><msqrt><mn>2</mn></msqrt></mtd></mtr></mtable></mfenced></math>Cộng từng vế hai phương trình của hệ mới, ta được <mn>6</mn><mi>x</mi><msqrt><mn>3</mn></msqrt><mo>=</mo><mn>3</mn><msqrt><mn>2</mn></msqrt></math>" id="MathJax-Element-11-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">6<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x√3=3√2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mn>6</mn><mi>�</mi><msqrt><mn>3</mn></msqrt><mo>=</mo><mn>3</mn><msqrt><mn>2</mn></msqrt></math>. Suy ra <mi>x</mi><mo>=</mo><mfrac><msqrt><mn>6</mn></msqrt><mn>6</mn></mfrac><mo>.</mo></math>" id="MathJax-Element-12-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=√66.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mfrac><msqrt><mn>6</mn></msqrt><mn>6</mn></mfrac><mo>.</mo></math>Thay <mi>x</mi><mo>=</mo><mfrac><msqrt><mn>6</mn></msqrt><mn>6</mn></mfrac></math>" id="MathJax-Element-13-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=√66<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mfrac><msqrt><mn>6</mn></msqrt><mn>6</mn></mfrac></math> vào phương trình thứ hai của hệ đã cho, ta được <mfrac><msqrt><mn>6</mn></msqrt><mn>6</mn></mfrac><mo>&#x22C5;</mo><msqrt><mn>6</mn></msqrt><mo>&#x2212;</mo><mi>y</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>2</mn></math>" id="MathJax-Element-14-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">√66⋅√6−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y√2=2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><msqrt><mn>6</mn></msqrt><mn>6</mn></mfrac><mo>⋅</mo><msqrt><mn>6</mn></msqrt><mo>−</mo><mi>�</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>2</mn></math>. Do đó <mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><msqrt><mn>2</mn></msqrt><mn>2</mn></mfrac><mo>.</mo></math>" id="MathJax-Element-15-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−√22.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><msqrt><mn>2</mn></msqrt><mn>2</mn></mfrac><mo>.</mo></math>Vậy hệ phương trình có nghiệm duy nhất <mfenced><mrow><mo>&#x2212;</mo><mfrac><msqrt><mn>2</mn></msqrt><mn>2</mn></mfrac><mo>;</mo><mtext>&#x2009;&#x2009;</mtext><mfrac><msqrt><mn>6</mn></msqrt><mn>6</mn></mfrac></mrow></mfenced><mo>.</mo></math>" id="MathJax-Element-16-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">(−√22;√66).<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced><mrow><mo>−</mo><mfrac><msqrt><mn>2</mn></msqrt><mn>2</mn></mfrac><mo>;</mo><mtext>  </mtext><mfrac><msqrt><mn>6</mn></msqrt><mn>6</mn></mfrac></mrow></mfenced><mo>.</mo></math>d) <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mfenced><mrow><mi>x</mi><mo>+</mo><mi>y</mi></mrow></mfenced><mo>+</mo><mn>3</mn><mfenced><mrow><mi>x</mi><mo>&#x2212;</mo><mi>y</mi></mrow></mfenced><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mfenced><mrow><mi>x</mi><mo>+</mo><mi>y</mi></mrow></mfenced><mo>+</mo><mn>2</mn><mfenced><mrow><mi>x</mi><mo>&#x2212;</mo><mi>y</mi></mrow></mfenced><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-17-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y)+3(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y)=4(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y)+2(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y)=5<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mfenced><mrow><mi>�</mi><mo>+</mo><mi>�</mi></mrow></mfenced><mo>+</mo><mn>3</mn><mfenced><mrow><mi>�</mi><mo>−</mo><mi>�</mi></mrow></mfenced><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mfenced><mrow><mi>�</mi><mo>+</mo><mi>�</mi></mrow></mfenced><mo>+</mo><mn>2</mn><mfenced><mrow><mi>�</mi><mo>−</mo><mi>�</mi></mrow></mfenced><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>x</mi><mo>+</mo><mn>2</mn><mi>y</mi><mo>+</mo><mn>3</mn><mi>x</mi><mo>&#x2212;</mo><mn>3</mn><mi>y</mi><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mi>x</mi><mo>+</mo><mi>y</mi><mo>+</mo><mn>2</mn><mi>x</mi><mo>&#x2212;</mo><mn>2</mn><mi>y</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-18-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y+3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>+</mo><mn>3</mn><mi>�</mi><mo>−</mo><mn>3</mn><mi>�</mi><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>−</mo><mn>2</mn><mi>�</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>5</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>x</mi><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-19-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=43<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>5</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>4</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>5</mn></mtd></mtr></mtable></mfenced></math>Trừ từng vế phương trình thứ nhất cho phương trình thứ hai của hệ mới, ta được2x = −1. Suy ra x = <mo>-</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></math>" id="MathJax-Element-20-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">−12<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>-</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></math>.Thay <mi>x</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></math>" id="MathJax-Element-21-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=−12<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></math> vào phương trình thứ hai của hệ mới, ta được <mn>3</mn><mo>&#x22C5;</mo><mfenced><mrow><mo>&#x2212;</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></mrow></mfenced><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>5</mn></math>" id="MathJax-Element-22-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">3⋅(−12)−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mn>3</mn><mo>⋅</mo><mfenced><mrow><mo>−</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></mrow></mfenced><mo>−</mo><mi>�</mi><mo>=</mo><mn>5</mn></math> hay <mfrac><mrow><mo>&#x2212;</mo><mn>3</mn></mrow><mn>2</mn></mfrac><mo>&#x2212;</mo><mi>y</mi><mo>=</mo><mn>5</mn></math>" id="MathJax-Element-23-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">−32−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=5<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mo>−</mo><mn>3</mn></mrow><mn>2</mn></mfrac><mo>−</mo><mi>�</mi><mo>=</mo><mn>5</mn></math>. Do đó <mi>y</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>13</mn><mn>2</mn></mfrac><mo>.</mo></math>" id="MathJax-Element-24-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=−132.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>13</mn><mn>2</mn></mfrac><mo>.</mo></math>Vậy hệ phương trình có nghiệm duy nhất <mfenced><mrow><mo>&#x2212;</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mo>;</mo><mtext>&#x2009;&#x2009;</mtext><mo>&#x2212;</mo><mfrac><mn>13</mn><mn>2</mn></mfrac></mrow></mfenced><mo>.</mo></math>" id="MathJax-Element-25-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">(−12;−132).Bài 3 trang 21 Toán 9 Tập 1: Xác định a, b để đồ thị hàm số y = ax + b đi qua hai điểm A và B trong mỗi trường hợp sau:a) A(1; 2) và B(3; 8);b) A(2; 1) và B(4; –2).Lời giải:a) • Đồ thị hàm số y = ax + b đi qua điểm A(1; 2) nên2 = a . 1 + b hay a + b = 2.      (1)• Đồ thị hàm số y = ax + b đi qua điểm B(3; 8) nên8 = a . 3 + b hay 3a + b = 8.      (2)Từ (1) và (2) ta có hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>a</mi><mo>+</mo><mi>b</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>a</mi><mo>+</mo><mi>b</mi><mo>=</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">b=23<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">b=8<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>2</mn></mtd></mtr><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>8</mn></mtd></mtr></mtable></mfenced></math>(I)Trừ từng vế hai phương trình của hệ (I), ta được –2a = –6. Suy ra a = 3.Thay a = 3 vào phương trình thứ nhất của hệ (I), ta được 3 + b = 2. Do đó b = –1.Vậy a = 3; b = –1.b) • Đồ thị hàm số y = ax + b đi qua điểm A(2; 1) nên1 = a . 2 + b hay 2a + b = 1.      (1)• Đồ thị hàm số y = ax + b đi qua điểm B(4; –2) nên–2 = a . 4 + b hay 4a + b = –2.   (2)Từ (1) và (2) ta có hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>a</mi><mo>+</mo><mi>b</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mn>4</mn><mi>a</mi><mo>+</mo><mi>b</mi><mo>=</mo><mo>&#x2212;</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">b=14<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">b=−2<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mn>4</mn><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mo>−</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math>(I)Trừ từng vế hai phương trình của hệ (I), ta được –2a = 3. Suy ra <mi>a</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a=−32<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></math>.Thay <mi>a</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a=−32<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></math> vào phương trình thứ nhất của hệ (I), ta được <mn>2</mn><mo>&#x22C5;</mo><mfenced><mrow><mo>&#x2212;</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></mrow></mfenced><mo>+</mo><mi>b</mi><mo>=</mo><mn>1</mn></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">2⋅(−32)+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">b=1<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mn>2</mn><mo>⋅</mo><mfenced><mrow><mo>−</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></mrow></mfenced><mo>+</mo><mi>�</mi><mo>=</mo><mn>1</mn></math> hay –3 + b = 1. Do đó b = 4.Vậy <mi>a</mi><mo>=</mo><mo>&#x2212;</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></math>" id="MathJax-Element-6-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">a=−32<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mo>=</mo><mo>−</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></math>; b = 4.Bài 4 trang 21 Toán 9 Tập 1: Trong tháng thứ nhất, hai tổ sản xuất được 800 chi tiết máy. So với tháng thứ nhất, trong tháng thứ hai, tổ một sản xuất vược 15%, tổ hai sản xuất vượt 20% nên trong tháng này, cả hai tổ đã sản xuất được 945 chi tiết máy. Hỏi trong tháng thứ nhất mỗi tổ sản xuất được bao nhiêu chi tiết máy?Lời giải:Gọi x, y lần lượt là số chi tiết máy trong tháng thứ nhất mỗi tổ sản xuất được (x, y ∈ ℕ*).Trong tháng thứ nhất, hai tổ sản xuất được 800 chi tiết máy nên ta có x + y  = 800. (1)Trong tháng thứ hai, tổ một sản xuất được là:x + 15%x = 115%x = 1,15x (chi tiết máy)Trong tháng thứ hai, tổ hai sản xuất được là:y + 20%y = 120%y = 1,2y (chi tiết máy).Theo đề bài, ta có phương trình 1,15x + 1,2y = 945.         (2)Từ (1) và (2) ta có hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>+</mo><mi>y</mi><mo>&#xA0;</mo><mo>=</mo><mn>800</mn></mtd></mtr><mtr><mtd><mn>1,15</mn><mi>x</mi><mo>+</mo><mn>1,2</mn><mi>y</mi><mo>=</mo><mn>945</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y =8001,15<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+1,2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=945<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo> </mo><mo>=</mo><mn>800</mn></mtd></mtr><mtr><mtd><mn>1,15</mn><mi>�</mi><mo>+</mo><mn>1,2</mn><mi>�</mi><mo>=</mo><mn>945</mn></mtd></mtr></mtable></mfenced></math>                                                         <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>&#xA0;</mo><mo>=</mo><mn>800</mn><mo>&#x2212;</mo><mi>y</mi></mtd></mtr><mtr><mtd><mn>1,15</mn><mfenced><mrow><mn>800</mn><mo>&#x2212;</mo><mi>y</mi></mrow></mfenced><mo>+</mo><mn>1,2</mn><mi>y</mi><mo>=</mo><mn>945</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x =800−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y1,15(800−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y)+1,2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=945<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo> </mo><mo>=</mo><mn>800</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mn>1,15</mn><mfenced><mrow><mn>800</mn><mo>−</mo><mi>�</mi></mrow></mfenced><mo>+</mo><mn>1,2</mn><mi>�</mi><mo>=</mo><mn>945</mn></mtd></mtr></mtable></mfenced></math>                                                         <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>&#xA0;</mo><mo>=</mo><mn>800</mn><mo>&#x2212;</mo><mi>y</mi></mtd></mtr><mtr><mtd><mn>920</mn><mo>&#x2212;</mo><mn>1,15</mn><mi>y</mi><mo>+</mo><mn>1,2</mn><mi>y</mi><mo>=</mo><mn>945</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x =800−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y920−1,15<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y+1,2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=945<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo> </mo><mo>=</mo><mn>800</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mn>920</mn><mo>−</mo><mn>1,15</mn><mi>�</mi><mo>+</mo><mn>1,2</mn><mi>�</mi><mo>=</mo><mn>945</mn></mtd></mtr></mtable></mfenced></math>                                                         <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>800</mn><mo>&#x2212;</mo><mi>y</mi></mtd></mtr><mtr><mtd><mn>0,05</mn><mi>y</mi><mo>=</mo><mn>25</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=800−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y0,05<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=25<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>800</mn><mo>−</mo><mi>�</mi></mtd></mtr><mtr><mtd><mn>0,05</mn><mi>�</mi><mo>=</mo><mn>25</mn></mtd></mtr></mtable></mfenced></math>                                                         <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>300</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>&#xA0;</mo><mo>=</mo><mn>500</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=300<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y =500<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>300</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo> </mo><mo>=</mo><mn>500</mn></mtd></mtr></mtable></mfenced></math> (thỏa mãn).Vậy trong tháng thứ nhất, tổ một sản xuất được 300 chi tiết máy, tổ hai sản xuất được 500 chi tiết máy.Bài 5 trang 21 Toán 9 Tập 1: Hai tổ sản xuất cùng may một loại áo khoác xuất khẩu. Nếu tổ thứ nhất may trong 7 ngày và tổ thứ hai may trong 5 ngày thì cả hai tổ may được 1540 chiếc áo. Biết rằng mỗi ngày tổ thứ hai may được nhiều hơn tổ thứ nhất 20 chiếc áo. Hỏi trong một ngày mỗi tổ may được bao nhiêu chiếc áo? (Năng suất may áo của mỗi tổ trong các ngày là như nhau.)Lời giải:Gọi x, y (chiếc) lần lượt là số áo trong một ngày mỗi tổ may được (x ∈ ℕ*, y ∈ ℕ*).Số chiếc áo tổ thứ nhất may trong 7 ngày là: 7x (chiếc)Số chiếc áo tổ thứ hai may trong 5 ngày là: 5y (chiếc)Theo đề bài ta có phương trình 7x + 5y = 1540.  (1)Mỗi ngày tổ thứ hai may được nhiều hơn tổ thứ nhất 20 chiếc áo nên ta cóy – x = 20.   (2)Từ (1) và (2) ta có hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>7</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>y</mi><mo>=</mo><mn>1540</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>&#x2212;</mo><mi>x</mi><mo>=</mo><mn>20</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{7<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=1540<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y−<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=20<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>7</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>=</mo><mn>1540</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>−</mo><mi>�</mi><mo>=</mo><mn>20</mn></mtd></mtr></mtable></mfenced></math>                                                          <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>7</mn><mi>x</mi><mo>+</mo><mn>5</mn><mfenced><mrow><mi>x</mi><mo>+</mo><mn>20</mn></mrow></mfenced><mo>=</mo><mn>1</mn><mtext>&#x2009;&#x2009;</mtext><mn>540</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mi>x</mi><mo>+</mo><mn>20</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{7<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5(<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+20)=1540<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+20<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>7</mn><mi>�</mi><mo>+</mo><mn>5</mn><mfenced><mrow><mi>�</mi><mo>+</mo><mn>20</mn></mrow></mfenced><mo>=</mo><mn>1</mn><mtext>  </mtext><mn>540</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mi>�</mi><mo>+</mo><mn>20</mn></mtd></mtr></mtable></mfenced></math>                                                          <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>7</mn><mi>x</mi><mo>+</mo><mn>5</mn><mi>x</mi><mo>+</mo><mn>100</mn><mo>=</mo><mn>1</mn><mtext>&#x2009;&#x2009;</mtext><mn>540</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mi>x</mi><mo>+</mo><mn>20</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{7<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+5<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+100=1540<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+20<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>7</mn><mi>�</mi><mo>+</mo><mn>5</mn><mi>�</mi><mo>+</mo><mn>100</mn><mo>=</mo><mn>1</mn><mtext>  </mtext><mn>540</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mi>�</mi><mo>+</mo><mn>20</mn></mtd></mtr></mtable></mfenced></math>                                                          <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>12</mn><mi>x</mi><mo>=</mo><mn>1</mn><mtext>&#x2009;&#x2009;</mtext><mn>440</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mi>x</mi><mo>+</mo><mn>20</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=1440<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+20<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>12</mn><mi>�</mi><mo>=</mo><mn>1</mn><mtext>  </mtext><mn>440</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mi>�</mi><mo>+</mo><mn>20</mn></mtd></mtr></mtable></mfenced></math>                                                          <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>120</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>140</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=120<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=140<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>120</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mn>140</mn></mtd></mtr></mtable></mfenced></math> (thỏa mãn điều kiện).Vậy trong một ngày, tổ một may được 120 chiếc áo và tổ hai may được 140 chiếc áo.Bài 6 trang 21 Toán 9 Tập 1: Trên một cánh đồng, người ta cấy 60 ha lúa giống mới và 40 ha lúa giống cũ, thu hoạch được tất cả 660 tấn thóc. Hỏi năng suất lúa giống mới trên 1 ha bằng bao nhiêu? Biết rằng 3 ha trồng lúa giống mới thu hoạch được ít hơn 4 ha trồng lúa giống cũ là 3 tấn.Lời giải:Gọi x, y (tấn thóc) lần lượt là năng suất lúa giống mới và năng suất lúa giống cũ trên 1 ha (x > 0, y > 0).Số tấn thóc khi cấy 60 ha lúa giống mới là 60x (tấn thóc).Số tấn thóc khi cấy 40 ha lúa giống cũ là 40x (tấn thóc).Theo bài, người ta thu hoạch được tất cả 660 tấn thóc nên60x + 40y = 660 hay 3x + 2y = 33.  (1)Với 3 ha trồng lúa giống mới thu hoạch được ít hơn 4 ha trồng lúa giống cũ là 3 tấn nên ta có 4y – 3x = 3 hay – 3x + 4y = 3.  (2)Từ (1) và (2) ta có hệ phương trình <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>x</mi><mo>+</mo><mn>2</mn><mi>y</mi><mo>=</mo><mn>33</mn></mtd></mtr><mtr><mtd><mo>&#x2212;</mo><mn>3</mn><mi>x</mi><mo>+</mo><mn>4</mn><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+2<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=33−3<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+4<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=3<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mn>3</mn><mi>�</mi><mo>+</mo><mn>2</mn><mi>�</mi><mo>=</mo><mn>33</mn></mtd></mtr><mtr><mtd><mo>−</mo><mn>3</mn><mi>�</mi><mo>+</mo><mn>4</mn><mi>�</mi><mo>=</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math>.Cộng từng vế hai phương trình của hệ mới, ta được 6y = 36. Suy ra y = 6 (thỏa mãn).Thay y = 6 vào phương trình thứ nhất, ta được 3x + 2 . 6 = 33. Do đó x = 7 (thỏa mãn).Vậy năng suất lúa giống mới trên 1 ha bằng 7 tấn/ha.Bài 7 trang 21 Toán 9 Tập 1: Cân bằng các phương trình hóa học sau bằng phương pháp đại số.a) Ag + Cl2 → AgClb) CO2 + C → COLời giải:a) Gọi x, y lần lượt là hệ số của Ag và Cl2 thỏa mãn cân bằng phương trình hóa họcxAg + yCl2 → AgClCân bằng số nguyên tử Ag, số nguyên tử Cl ở hai vế, ta được <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>y</mi><mo>=</mo><mn>1.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-1-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=1.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>=</mo><mn>1.</mn></mtd></mtr></mtable></mfenced></math>Giải hệ phương trình trên, ta được <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mo>.</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-2-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=1<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=12.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mo>.</mo></mtd></mtr></mtable></mfenced></math>Đưa các hệ số tìm được vào phương trình hóa học, ta có<mi>A</mi><mi>g</mi><mo>+</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mi>C</mi><msub><mi>l</mi><mn>2</mn></msub><mo>&#x2192;</mo><mi>A</mi><mi>g</mi><mi>C</mi><mi>l</mi></math>" id="MathJax-Element-3-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0"><span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.507em; white-space: pre;">A<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.003em; padding-top: 0.232em; white-space: pre;">g+12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-right: 0.045em; padding-top: 0.461em; white-space: pre;">C<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">l2→<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.507em; white-space: pre;">A<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.003em; padding-top: 0.232em; white-space: pre;">g<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-right: 0.045em; padding-top: 0.461em; white-space: pre;">C<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">l<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>�</mi><mi>�</mi><mo>+</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mi>�</mi><msub><mi>�</mi><mn>2</mn></msub><mo>→</mo><mi>�</mi><mi>�</mi><mi>�</mi><mi>�</mi></math>Do các hệ số của phương trình hóa học phải là số nguyên nên nhân hai vế của phương trình hóa học với 2, ta được2Ag + yCl2 → 2AgClb) Gọi x, y lần lượt là hệ số của CO2 và C thỏa mãn cân bằng phương trình hóa họcxCO2 + yC → COCân bằng số nguyên tử C, số nguyên tử O ở hai vế, ta được <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>+</mo><mi>y</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>x</mi><mo>=</mo><mn>1.</mn></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-4-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x+<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=1.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>+</mo><mi>�</mi><mo>=</mo><mn>1</mn></mtd></mtr><mtr><mtd><mn>2</mn><mi>�</mi><mo>=</mo><mn>1.</mn></mtd></mtr></mtable></mfenced></math>Giải hệ phương trình trên, ta được: <mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>y</mi><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></mtd></mtr><mtr><mtd><mi>x</mi><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mo>.</mo></mtd></mtr></mtable></mfenced></math>" id="MathJax-Element-5-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">{<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.461em; padding-right: 0.006em; padding-top: 0.232em; white-space: pre;">y=12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.232em; white-space: pre;">x=12.<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced close="" open="{"><mtable columnalign="left"><mtr><mtd><mi>�</mi><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></mtd></mtr><mtr><mtd><mi>�</mi><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mo>.</mo></mtd></mtr></mtable></mfenced></math>Đưa các hệ số tìm được vào phương trình hóa học, ta có<mfrac><mn>1</mn><mn>2</mn></mfrac><mi>C</mi><msub><mi>O</mi><mn>2</mn></msub><mo>+</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mi>C</mi><mo>&#x2192;</mo><mi>C</mi><mi>O</mi></math>" id="MathJax-Element-6-Frame" role="presentation" style="border: 0px; box-sizing: border-box; direction: ltr; display: inline-block; float: none; line-height: 0; margin: 0px; max-height: none; max-width: none; min-height: 0px; min-width: 0px; overflow-wrap: normal; padding: 1px 0px; position: relative; text-align: left; white-space: nowrap; word-spacing: normal;" tabindex="0">12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-right: 0.045em; padding-top: 0.461em; white-space: pre;">C<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">O2+12<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-right: 0.045em; padding-top: 0.461em; white-space: pre;">C→<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-right: 0.045em; padding-top: 0.461em; white-space: pre;">C<span class="mjx-char MJXc-TeX-math-I" face="MJXc-TeX-math-I, MJXc-TeX-math-Ix, MJXc-TeX-math-Iw" style="box-sizing: content-box; display: block; padding-bottom: 0.278em; padding-top: 0.461em; white-space: pre;">O<span class="MJX_Assistive_MathML" role="presentation" style="border: 0px; box-sizing: border-box; clip: rect(1px, 1px, 1px, 1px); display: block; height: 1px; left: 0px; overflow: hidden; padding: 1px 0px 0px; position: absolute; top: 0px; user-select: none; width: 1px;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>1</mn><mn>2</mn></mfrac><mi>�</mi><msub><mi>�</mi><mn>2</mn></msub><mo>+</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mi>�</mi><mo>→</mo><mi>�</mi><mi>�</mi></math>Do các hệ số của phương trình hóa học phải là số nguyên nên nhân hai vế của phương trình hóa học với 2, ta đượcCO2 + C → 2CO

C1 Bài 3: Giải hệ hai phương trình bậc nhất hai ẩn

Khởi động trang 15 Toán 9 Tập 1: Tại một cửa hàng, chị An mua 1,2 kg thịt lợn và 0,7 kg thịt bò hết 362 000 đồng; chị Ba mua 0,8 kg thịt ...

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