Ôn tập chương IV
Hướng dẫn giải Bài 59 (Trang 63 SGK Toán Đại số 9, Tập 2)
<p>Giải phương tr&igrave;nh bằng c&aacute;ch đặt ẩn phụ:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>a</mi><mo>)</mo><mo>&#160;</mo><mn>2</mn><msup><mfenced><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi></mrow></mfenced><mn>2</mn></msup><mo>+</mo><mn>3</mn><mfenced><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi></mrow></mfenced><mo>+</mo><mn>1</mn><mo>=</mo><mn>0</mn><mspace linebreak="newline"/><mi>b</mi><mo>)</mo><mo>&#160;</mo><msup><mfenced><mrow><mi>x</mi><mo>+</mo><mfrac><mn>1</mn><mi>x</mi></mfrac></mrow></mfenced><mn>2</mn></msup><mo>-</mo><mn>4</mn><mfenced><mrow><mi>x</mi><mo>+</mo><mfrac><mn>1</mn><mi>x</mi></mfrac></mrow></mfenced><mo>+</mo><mn>3</mn><mo>=</mo><mn>0</mn></math></p> <p><strong>Giải:&nbsp;</strong></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>a</mi><mo>)</mo><mo>&#160;</mo><mn>2</mn><msup><mfenced><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi></mrow></mfenced><mn>2</mn></msup><mo>+</mo><mn>3</mn><mfenced><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi></mrow></mfenced><mo>+</mo><mn>1</mn><mo>=</mo><mn>0</mn></math></p> <p>Đặt <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi><mo>=</mo><mi>t</mi></math>, ta c&oacute;: <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>2</mn><msup><mi>t</mi><mn>2</mn></msup><mo>+</mo><mn>3</mn><mi>t</mi><mo>+</mo><mn>1</mn><mo>=</mo><mn>0</mn></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>t</mi><mn>1</mn></msub><mo>=</mo><mo>-</mo><mn>1</mn><mo>,</mo><mo>&#160;</mo><msub><mi>t</mi><mn>2</mn></msub><mo>=</mo><mo>-</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></math></p> <p>- Với <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>t</mi><mn>1</mn></msub><mo>=</mo><mo>-</mo><mn>1</mn></math>, ta c&oacute;: <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi><mo>=</mo><mo>-</mo><mn>1</mn><mo>&#8660;</mo><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi><mo>+</mo><mn>1</mn><mo>=</mo><mn>0</mn></math></p> <p>Phương tr&igrave;nh c&oacute; nghiệm k&eacute;p:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>x</mi><mn>1</mn></msub><mo>=</mo><msub><mi>x</mi><mn>2</mn></msub><mo>=</mo><mn>1</mn></math></p> <p>- Với <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>t</mi><mn>2</mn></msub><mo>=</mo><mo>-</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></math>, ta c&oacute; <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi><mo>=</mo><mo>-</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mo>&#8660;</mo><mn>2</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>4</mn><mi>x</mi><mo>+</mo><mn>1</mn><mo>=</mo><mn>0</mn></math></p> <p>Phương tr&igrave;nh c&oacute; hai nghiệm ph&acirc;n biệt:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>x</mi><mn>3</mn></msub><mo>=</mo><mfrac><mrow><mn>2</mn><mo>+</mo><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>4</mn></msub><mo>=</mo><mfrac><mrow><mn>2</mn><mo>-</mo><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac></math></p> <p>Vậy phương tr&igrave;nh c&oacute; ba nghiệm:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>x</mi><mn>1</mn></msub><mo>=</mo><msub><mi>x</mi><mn>2</mn></msub><mo>=</mo><mn>1</mn><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>3</mn></msub><mo>=</mo><mfrac><mrow><mn>2</mn><mo>+</mo><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>4</mn></msub><mo>=</mo><mfrac><mrow><mn>2</mn><mo>-</mo><msqrt><mn>2</mn></msqrt></mrow><mn>2</mn></mfrac></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>b</mi><mo>)</mo><mo>&#160;</mo><msup><mfenced><mrow><mi>x</mi><mo>+</mo><mfrac><mn>1</mn><mi>x</mi></mfrac></mrow></mfenced><mn>2</mn></msup><mo>-</mo><mn>4</mn><mfenced><mrow><mi>x</mi><mo>+</mo><mfrac><mn>1</mn><mi>x</mi></mfrac></mrow></mfenced><mo>+</mo><mn>3</mn><mo>=</mo><mn>0</mn></math>. Điều kiện: <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi><mo>&#8800;</mo><mn>0</mn></math></p> <p>Đặt&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi><mo>+</mo><mfrac><mn>1</mn><mi>x</mi></mfrac><mo>=</mo><mi>t</mi></math>, ta c&oacute;:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>t</mi><mn>2</mn></msup><mo>-</mo><mn>4</mn><mi>t</mi><mo>+</mo><mn>3</mn><mo>=</mo><mn>0</mn><mo>&#8658;</mo><msub><mi>t</mi><mn>1</mn></msub><mo>=</mo><mn>1</mn><mo>,</mo><mo>&#160;</mo><msub><mi>t</mi><mn>2</mn></msub><mo>=</mo><mn>3</mn></math></p> <p>- Với <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>t</mi><mn>1</mn></msub><mo>=</mo><mn>1</mn></math>, ta c&oacute;:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi><mo>+</mo><mfrac><mn>1</mn><mi>x</mi></mfrac><mo>=</mo><mn>1</mn><mo>&#8660;</mo><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mi>x</mi><mo>+</mo><mn>1</mn><mo>=</mo><mn>0</mn></math></p> <p>Phương tr&igrave;nh v&ocirc; nghiệm.</p> <p>- Với <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>t</mi><mn>2</mn></msub><mo>=</mo><mn>3</mn></math>, ta c&oacute;: <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi><mo>+</mo><mfrac><mn>1</mn><mi>x</mi></mfrac><mo>=</mo><mn>3</mn><mo>&#8660;</mo><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>3</mn><mi>x</mi><mo>+</mo><mn>1</mn><mo>=</mo><mn>0</mn></math></p> <p>Phương tr&igrave;nh c&oacute; hai nghiệm <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>x</mi><mn>1</mn></msub><mo>=</mo><mfrac><mrow><mn>3</mn><mo>+</mo><msqrt><mn>5</mn></msqrt></mrow><mn>2</mn></mfrac><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>2</mn></msub><mo>=</mo><mfrac><mrow><mn>3</mn><mo>-</mo><msqrt><mn>5</mn></msqrt></mrow><mn>2</mn></mfrac></math></p>
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