Bài 4: Giải hệ phương trình bằng phương pháp cộng đại số
Hướng dẫn giải Bài 23 (Trang 19 SGK Toán Đại số 9, Tập 2)
<p>Giải hệ phương tr&igrave;nh sau:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mo>(</mo><mn>1</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>x</mi><mo>+</mo><mo>(</mo><mn>1</mn><mo>-</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>y</mi><mo>=</mo><mn>5</mn></mtd></mtr><mtr><mtd><mo>(</mo><mn>1</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>x</mi><mo>+</mo><mo>(</mo><mn>1</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>y</mi><mo>=</mo><mn>3</mn></mtd></mtr></mtable></mfenced></math></p> <p>Giải:</p> <p>Ta c&oacute;:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mo>(</mo><mn>1</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>x</mi><mo>+</mo><mo>(</mo><mn>1</mn><mo>-</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>y</mi><mo>=</mo><mn>5</mn><mo>&#160;</mo><mo>&#160;</mo><mo>&#160;</mo><mo>&#160;</mo><mo>(</mo><mn>1</mn><mo>)</mo></mtd></mtr><mtr><mtd><mo>(</mo><mn>1</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>x</mi><mo>+</mo><mo>(</mo><mn>1</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>y</mi><mo>=</mo><mn>3</mn><mo>&#160;</mo><mo>&#160;</mo><mo>&#160;</mo><mo>&#160;</mo><mo>&#160;</mo><mo>(</mo><mn>2</mn><mo>)</mo></mtd></mtr></mtable></mfenced></math></p> <p>Trừ từng vế hai phương tr&igrave;nh (1) v&agrave; (2) ta được:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>(</mo><mn>1</mn><mo>-</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>y</mi><mo>-</mo><mo>(</mo><mn>1</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>y</mi><mo>=</mo><mn>2</mn><mspace linebreak="newline"/><mo>&#8660;</mo><mo>(</mo><mn>1</mn><mo>-</mo><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>1</mn><mo>-</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>y</mi><mo>=</mo><mn>2</mn><mo>&#8660;</mo><mo>-</mo><mn>2</mn><mi>y</mi><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>2</mn><mspace linebreak="newline"/><mo>&#8660;</mo><mi>y</mi><mo>=</mo><mfrac><mrow><mo>-</mo><mn>2</mn></mrow><mrow><mn>2</mn><msqrt><mn>2</mn></msqrt></mrow></mfrac><mo>&#8660;</mo><mi>y</mi><mo>=</mo><mo>-</mo><mfrac><mn>1</mn><msqrt><mn>2</mn></msqrt></mfrac><mo>&#8660;</mo><mi>y</mi><mo>=</mo><mfrac><mrow><mo>-</mo><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac><mo>&#160;</mo><mo>&#160;</mo><mo>&#160;</mo><mo>(</mo><mn>3</mn><mo>)</mo></math></p> <p>Thay (3) v&agrave;o (1) ta được:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8660;</mo><mo>(</mo><mn>1</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>x</mi><mo>+</mo><mo>(</mo><mn>1</mn><mo>-</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mo>(</mo><mfrac><mrow><mo>-</mo><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac><mo>)</mo><mo>=</mo><mn>5</mn><mspace linebreak="newline"/><mo>&#8660;</mo><mo>(</mo><mn>1</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mi>x</mi><mo>=</mo><mfrac><mrow><mn>8</mn><mo>+</mo><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac><mo>&#8660;</mo><mi>x</mi><mo>=</mo><mfrac><mrow><mn>8</mn><mo>+</mo><msqrt><mn>2</mn></msqrt></mrow><mrow><mn>2</mn><mo>(</mo><mn>1</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo></mrow></mfrac><mspace linebreak="newline"/><mo>&#8660;</mo><mi>x</mi><mo>=</mo><mfrac><mrow><mo>(</mo><mn>8</mn><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>)</mo><mo>(</mo><mn>1</mn><mo>-</mo><msqrt><mn>2</mn></msqrt><mo>)</mo></mrow><mrow><mn>2</mn><mo>(</mo><mn>1</mn><mo>-</mo><mn>2</mn><mo>)</mo></mrow></mfrac><mo>&#8660;</mo><mfrac><mrow><mn>8</mn><mo>-</mo><mn>8</mn><msqrt><mn>2</mn></msqrt><mo>+</mo><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>2</mn></mrow><mrow><mo>-</mo><mn>2</mn></mrow></mfrac><mspace linebreak="newline"/><mo>&#8660;</mo><mi>x</mi><mo>=</mo><mo>-</mo><mfrac><mrow><mn>6</mn><mo>-</mo><mn>7</mn><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac><mo>&#8660;</mo><mfrac><mrow><mo>-</mo><mn>6</mn><mo>+</mo><mn>7</mn><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac><mspace linebreak="newline"/><mi>H</mi><mi>&#7879;</mi><mo>&#160;</mo><mi>c</mi><mi>&#243;</mi><mo>&#160;</mo><mi>n</mi><mi>g</mi><mi>h</mi><mi>i</mi><mi>&#7879;</mi><mi>m</mi><mo>&#160;</mo><mi>l</mi><mi>&#224;</mi><mo>&#160;</mo><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mfrac><mrow><mo>-</mo><mn>6</mn><mo>+</mo><mn>7</mn><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac></mtd></mtr><mtr><mtd><mi>x</mi><mo>=</mo><mfrac><mrow><mo>-</mo><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac></mtd></mtr></mtable></mfenced><mo>&#160;</mo></math></p>
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