Bài 7: Phương trình quy về phương trình bậc hai
Hướng dẫn giải Bài 39 (Trang 57 SGK Toán Đại số 9, Tập 2)
<p>Giải phương tr&igrave;nh bằng c&aacute;ch đưa về phương tr&igrave;nh t&iacute;ch.</p> <p>a) (3x<sup>2</sup> &minus; 7x &minus; 10)<math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="[" close="]"><mrow><mn>2</mn><msup><mi>x</mi><mn>2</mn></msup><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mo>(</mo><mn>1</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><msqrt><mn>5</mn></msqrt><mo>)</mo><mi>x</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msqrt><mn>5</mn></msqrt><mo>&#160;</mo><mo>&#8722;</mo><mn>3</mn></mrow></mfenced></math> = 0;</p> <p>b) x<sup>3</sup> + 3x<sup>2</sup> &minus; 2x &minus; 6 = 0 ;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;c) (x<sup>2</sup> &minus; 1)(0,6x + 1) = 0,6x<sup>2</sup> +x ;</p> <p>d) (x<sup>2</sup> + 2x &minus; 5)<sup>2</sup> = (x<sup>2</sup> &minus; x + 5)<sup>2</sup> .</p> <p><strong>Giải</strong></p> <p>a) (3x<sup>2</sup> &minus; 7x &minus; 10)<math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="[" close="]"><mrow><mn>2</mn><msup><mi>x</mi><mn>2</mn></msup><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mo>(</mo><mn>1</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><msqrt><mn>5</mn></msqrt><mo>)</mo><mi>x</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msqrt><mn>5</mn></msqrt><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>3</mn></mrow></mfenced></math> = 0</p> <p>&hArr; 3x<sup>2</sup> &minus; 7x &minus; 10 = 0 (1) hoặc 2x<sup>2</sup> + (1 &minus;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>5</mn></msqrt></math>)x +&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>5</mn></msqrt></math> &minus; 3 = 0 (2)</p> <p>Giải (1): Phương tr&igrave;nh c&oacute; a&nbsp;<strong>&minus;</strong> b + c = 3 + 7 &minus; 10 = 0</p> <p>n&ecirc;n x<sub>1</sub> = &minus;1, x<sub>2</sub> = &minus;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mo>&#8722;</mo><mn>10</mn></mrow><mn>3</mn></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>10</mn><mn>3</mn></mfrac></math>&nbsp;</p> <p>Giải (2): Phương tr&igrave;nh c&oacute; a + b + c = 2 + (1 &minus;<math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>5</mn></msqrt></math> ) +&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>5</mn></msqrt></math> &minus; 3 = 0</p> <p>n&ecirc;n x<sub>3&nbsp;</sub>= 1, x<sub>4&nbsp;</sub>=&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><msqrt><mn>5</mn></msqrt><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>3</mn></mrow><mn>2</mn></mfrac></math>&nbsp;</p> <p>b) x<sup>3</sup> + 3x<sup>2&nbsp;</sup>&minus; 2x &minus; 6 = 0 &hArr; x<sup>2</sup>(x + 3) &minus; 2(x + 3) = 0 &hArr; (x + 3)(x<sup>2</sup> &minus; 2) = 0</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &hArr; x + 3 = 0 (1) hoặc x<sup>2</sup> &minus; 2 = 0 (2)</p> <p>Giải (1) ta được: x<sub>1&nbsp;</sub>= &minus;3</p> <p>Giải (2) ta được: x<sub>2</sub> = &minus;<math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>2</mn></msqrt></math> , x<sub>3</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>2</mn></msqrt></math>&nbsp;</p> <p>c) (x<sup>2</sup> &minus; 1)(0,6x + 1) = 0,6x<sup>2&nbsp;</sup>+ x &hArr; (0,6x + 1)(x<sup>2</sup> &minus; x &minus; 1) = 0</p> <p>&hArr; 0,6x + 1 = 0 (1) hoặc x<sup>2</sup> &minus; x &minus; 1 = 0 (2)</p> <p>(1) &hArr; 0,6x + 1 = 0 &hArr; x<sub>1</sub> = &minus;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>1</mn><mrow><mn>0</mn><mo>,</mo><mn>6</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mfrac><mn>5</mn><mn>3</mn></mfrac></math>&nbsp;</p> <p>(2): ∆ = (&minus;1)<sup>2</sup> &minus; 4.1. (&minus;1) = 1 + 4 = 5,&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mo>&#8710;</mo></msqrt></math> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>5</mn></msqrt></math>&nbsp;</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; x<sub>2&nbsp;</sub>=&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mn>1</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><msqrt><mn>5</mn></msqrt></mrow><mn>2</mn></mfrac></math>, x<sub>3</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mn>1</mn><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msqrt><mn>5</mn></msqrt></mrow><mn>2</mn></mfrac></math></p> <p>Vậy phương tr&igrave;nh c&oacute; ba nghiệm</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;x<sub>1&nbsp;</sub>= &minus;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>5</mn><mn>3</mn></mfrac></math> , x<sub>2</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mn>1</mn><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msqrt><mn>5</mn></msqrt></mrow><mn>2</mn></mfrac></math> , x<sub>3</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mn>1</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><msqrt><mn>5</mn></msqrt></mrow><mn>2</mn></mfrac></math>&nbsp;</p> <p>d) (x<sup>2</sup> + 2x &minus; 5)<sup>2</sup> = (x<sup>2</sup> &minus; x + 5)<sup>2</sup> &nbsp;&hArr; (x<sup>2</sup> + 2x &minus; 5)<sup>2</sup> &minus; (x<sup>2</sup> &minus; x +5)<sup>2</sup> = 0</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&hArr; (x<sup>2</sup> + 2x &minus; 5 + x<sup>2</sup> &minus; x + 5)(x<sup>2</sup> + 2x<sup>2</sup> &minus; 5 &minus; x<sup>2</sup> + x &minus; 5) = 0</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&hArr; (2x<sup>2</sup> +x )(3x &minus; 10) = 0</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&hArr; x(2x + 1)(3x &minus; 10) = 0&nbsp;</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&hArr; x<sub>1&nbsp;</sub>= 0, x<sub>2&nbsp;</sub>= &minus;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>1</mn><mn>2</mn></mfrac></math>, x<sub>3</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>10</mn><mn>3</mn></mfrac></math></p> <p>Vậy phương tr&igrave;nh c&oacute; 3 nghiệm:</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;x<sub>1</sub> = 0, x<sub>2</sub> = &minus;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>1</mn><mn>2</mn></mfrac></math> , x<sub>3</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>10</mn><mn>3</mn></mfrac></math></p>
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