Ôn tập chương III
Hướng dẫn giải Bài 42 (Trang 27 SGK Toán Đại số 9, Tập 2)
<p>Giải hệ phương tr&igrave;nh&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mn>2</mn><mi>x</mi><mo>-</mo><mi>y</mi><mo>=</mo><mi>m</mi></mtd></mtr><mtr><mtd><mn>4</mn><mi>x</mi><mo>-</mo><msup><mi>m</mi><mn>2</mn></msup><mi>y</mi><mo>=</mo><mn>2</mn><msqrt><mn>2</mn></msqrt></mtd></mtr></mtable></mfenced></math> trong mỗi trường hợp sau:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>a</mi><mo>)</mo><mo>&#160;</mo><mi>m</mi><mo>=</mo><mo>-</mo><msqrt><mn>2</mn></msqrt><mo>&#160;</mo><mo>&#160;</mo><mo>&#160;</mo><mo>&#160;</mo><mspace linebreak="newline"/><mi>b</mi><mo>)</mo><mo>&#160;</mo><mi>m</mi><mo>=</mo><msqrt><mn>2</mn></msqrt><mspace linebreak="newline"/><mi>c</mi><mo>)</mo><mo>&#160;</mo><mi>m</mi><mo>=</mo><mn>1</mn></math></p> <p><strong>Giải:&nbsp;</strong></p> <p>C&aacute;ch 1: (D&ugrave;ng phương ph&aacute;p thế)</p> <p>Từ phương tr&igrave;nh đầu ta c&oacute; <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>-</mo><mi>m</mi></math>. Thế v&agrave;o phương tr&igrave;nh sau để khử ẩn y th&igrave; được</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mn>4</mn><mi>x</mi><mo>-</mo><msup><mi>m</mi><mn>2</mn></msup><mfenced><mrow><mn>2</mn><mi>x</mi><mo>-</mo><mi>m</mi></mrow></mfenced><mo>=</mo><mn>2</mn><msqrt><mn>2</mn></msqrt><mo>&#8660;</mo><mn>2</mn><mfenced><mrow><mn>2</mn><mo>-</mo><msup><mi>m</mi><mn>2</mn></msup></mrow></mfenced><mi>x</mi><mo>=</mo><mn>2</mn><msqrt><mn>2</mn></msqrt><mo>-</mo><msup><mi>m</mi><mn>3</mn></msup><mspace linebreak="newline"/><mo>&#8660;</mo><mn>2</mn><mfenced><mrow><msqrt><mn>2</mn></msqrt><mo>-</mo><mi>m</mi></mrow></mfenced><mfenced><mrow><msqrt><mn>2</mn></msqrt><mo>+</mo><mi>m</mi></mrow></mfenced><mi>x</mi><mo>=</mo><mfenced><mrow><msqrt><mn>2</mn></msqrt><mo>-</mo><mi>m</mi></mrow></mfenced><mfenced><mrow><mn>2</mn><mo>+</mo><mi>m</mi><msqrt><mn>2</mn></msqrt><mo>+</mo><msup><mi>m</mi><mn>2</mn></msup></mrow></mfenced><mo>&#160;</mo><mfenced><mn>1</mn></mfenced></math></p> <p>a) Với <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>m</mi><mo>=</mo><mo>-</mo><msqrt><mn>2</mn></msqrt></math>, phương tr&igrave;nh (1) trở th&agrave;nh <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>0</mn><mi>x</mi><mo>=</mo><mn>4</mn><msqrt><mn>2</mn></msqrt></math>, v&ocirc; nghiệm. Vậy hệ đ&atilde; cho v&ocirc; nghiệm.</p> <p>b) Với <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>m</mi><mo>=</mo><msqrt><mn>2</mn></msqrt></math>, (1) trở th&agrave;nh <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>0</mn><mi>x</mi><mo>=</mo><mn>0</mn></math>, đ&uacute;ng với mọi x.</p> <p>Hệ c&oacute; nghiệm <math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>&#8712;</mo><mi mathvariant="normal">&#8477;</mi></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mi>x</mi><mo>-</mo><msqrt><mn>2</mn></msqrt></mtd></mtr></mtable></mfenced></math></p> <p>c) Với <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>m</mi><mo>=</mo><mn>1</mn></math>, (1) trở th&agrave;nh <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>2</mn><mi>x</mi><mo>=</mo><mn>2</mn><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>1</mn><mo>&#8660;</mo><mi>x</mi><mo>=</mo><mfrac><mrow><mn>2</mn><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>1</mn></mrow><mn>2</mn></mfrac></math></p> <p>Hệ c&oacute; nghiệm <math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mfrac><mrow><mn>2</mn><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>1</mn></mrow><mn>2</mn></mfrac></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>2</mn></mtd></mtr></mtable></mfenced></math></p> <p>C&aacute;ch 2: (D&ugrave;ng phương ph&aacute;p c&ocirc;ng đại số): Nh&acirc;n hai vế của phương tr&igrave;nh đầu với - 2 r&ocirc;̂i cộng từng vế hai phương tr&igrave;nh th&igrave; được</p> <p>a) Với <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>m</mi><mo>=</mo><mo>-</mo><msqrt><mn>2</mn></msqrt></math>, phương tr&igrave;nh (2) trở th&agrave;nh <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>0</mn><mi>y</mi><mo>=</mo><mn>4</mn><msqrt><mn>2</mn></msqrt></math>, v&ocirc; nghiệm.</p> <p>Vậy hệ đ&atilde; cho v&ocirc; nghiệm.</p> <p>b) Với <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>m</mi><mo>=</mo><msqrt><mn>2</mn></msqrt></math>, (2) trở th&agrave;nh <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>0</mn><mi>y</mi><mo>=</mo><mn>0</mn></math>, đ&uacute;ng với mọi y.</p> <p>Hệ c&oacute; nghiệm <math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mfrac><mi>y</mi><mn>2</mn></mfrac><mo>+</mo><mfrac><msqrt><mn>2</mn></msqrt><mn>2</mn></mfrac></mtd></mtr><mtr><mtd><mi>y</mi><mo>&#8712;</mo><mi mathvariant="normal">&#8477;</mi></mtd></mtr></mtable></mfenced></math></p> <p>c) Với <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>m</mi><mo>=</mo><mn>1</mn></math>, (2) trở th&agrave;nh <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>y</mi><mo>=</mo><mn>2</mn><mfenced><mrow><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>1</mn></mrow></mfenced></math></p> <p>Hệ c&oacute; nghiệm <math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="{" close=""><mtable columnalign="left"><mtr><mtd><mi>x</mi><mo>=</mo><mfrac><mrow><mn>2</mn><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>1</mn></mrow><mn>2</mn></mfrac></mtd></mtr><mtr><mtd><mi>y</mi><mo>=</mo><mn>2</mn><mfenced><mrow><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>1</mn></mrow></mfenced></mtd></mtr></mtable></mfenced></math></p>
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