Bài 7: Phương trình quy về phương trình bậc hai
Hướng dẫn giải Bài 38 (Trang 56-57 SGK Toán Đại số 9, Tập 2)
<p>Giải c&aacute;c phương tr&igrave;nh:</p> <p>a) (x &minus; 3)<sup>2</sup> + (x + 4)<sup>2</sup> = 23 &minus; 3x ;</p> <p>b) x<sup>3</sup> + 2x<sup>2</sup> &minus; (x &minus; 3)<sup>2</sup> = (x &minus; 1)(x<sup>2</sup> &minus; 2) ;</p> <p>c) (x &minus; 1)<sup>3</sup> + 0,5x<sup>2</sup> = x(x<sup>2</sup> + 1,5) ;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;d)&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>x</mi><mo>(</mo><mi>x</mi><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>7</mn><mo>)</mo></mrow><mn>3</mn></mfrac><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>x</mi><mn>2</mn></mfrac><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mfrac><mrow><mi>x</mi><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>4</mn></mrow><mn>3</mn></mfrac></math> ;</p> <p>e)&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>14</mn><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>9</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mfrac><mn>1</mn><mrow><mn>3</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mi>x</mi></mrow></mfrac></math> ;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;f)&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mn>2</mn><mi>x</mi></mrow><mrow><mi>x</mi><mo>+</mo><mo>&#160;</mo><mn>1</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mi>x</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mn>8</mn></mrow><mrow><mo>(</mo><mi>x</mi><mo>&#160;</mo><mo>+</mo><mn>1</mn><mo>)</mo><mo>(</mo><mi>x</mi><mo>&#8722;</mo><mo>&#160;</mo><mn>4</mn><mo>)</mo></mrow></mfrac></math> .</p> <p><strong>Giải</strong></p> <p>a) (x &minus; 3)<sup>2</sup> + (x + 4)<sup>2</sup> = 23 &minus; 3x &hArr; x<sup>2</sup> &minus; 6x + 9 + x<sup>2</sup> + 8x + 16 = 23 &minus; 3x&nbsp;</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &hArr; 2x<sup>2</sup> + 5x + 2 = 0</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;∆ = 25 &minus; 16 = 9&nbsp;</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; x<sub>1</sub> = &minus;2, x<sub>2</sub> = &minus;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>1</mn><mn>2</mn></mfrac></math>&nbsp;</p> <p>b) x<sup>3</sup> + 2x<sup>2</sup> &minus; (x &minus; 3)<sup>2</sup> = (x &minus; 1)(x<sup>2</sup> &minus; 2)</p> <p>&hArr; x<sup>3</sup> + 2x<sup>2</sup> &minus; x<sup>2</sup> + 6x &minus; 9 = x<sup>3</sup> &minus; x<sup>2</sup> &minus; 2x + 2 &hArr; 2x<sup>2</sup> + 8x &minus; 11 = 0</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ∆' = 16 + 22 = 38</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; x<sub>1</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mo>&#8722;</mo><mn>4</mn><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msqrt><mn>38</mn></msqrt></mrow><mn>2</mn></mfrac></math> , x<sub>2</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mo>&#8722;</mo><mn>4</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><msqrt><mn>38</mn></msqrt></mrow><mn>2</mn></mfrac></math>&nbsp;</p> <p>c) (x &minus; 1)<sup>3</sup> + 0,5x<sup>2</sup> = x(x<sup>2</sup> + 1,5) &hArr; x<sup>3</sup> &minus; 3x<sup>2</sup> + 3x &minus; 1 + 0,5x<sup>2</sup> = x<sup>3</sup> + 1,5x</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &hArr; 2,5x<sup>2</sup> &minus; 1,5x + 1 = 0</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &hArr; 5x<sup>2</sup> &minus; 3x + 2 = 0 ; ∆ = 9 &minus; 40 = &minus; 31 &lt; 0</p> <p>Phương tr&igrave;nh v&ocirc; nghiệm.</p> <p>d)&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>x</mi><mo>(</mo><mi>x</mi><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>7</mn><mo>)</mo></mrow><mn>3</mn></mfrac><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>x</mi><mn>2</mn></mfrac><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mfrac><mrow><mi>x</mi><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>4</mn></mrow><mn>3</mn></mfrac></math> &hArr; 2x(x &minus; 7) &minus; 6 = 3x &minus; 2(x &minus; 4 )</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &hArr; 2x<sup>2</sup> &minus; 14x &minus; 6 = 3x &minus; 2x + 8</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &hArr; 2x<sup>2</sup> &minus; 15x &minus; 14 = 0; ∆ = 225 + 112 = 337</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; x<sub>1</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mn>15</mn><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msqrt><mn>337</mn></msqrt></mrow><mn>4</mn></mfrac></math> , x<sub>2</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mn>15</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><msqrt><mn>337</mn></msqrt></mrow><mn>4</mn></mfrac></math>&nbsp;</p> <p>e)&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>14</mn><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>9</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mfrac><mn>1</mn><mrow><mn>3</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mi>x</mi></mrow></mfrac></math> . Điều kiện: x &ne;<a title="Nguồn tham khảo từ b&agrave;i viết Dấu kh&aacute;c (&ne;) trong Excel ứng dụng v&agrave; c&aacute;ch viết được thực hiện bởi website Thủ Thuật Nhanh" href="https://thuthuatnhanh.com/cach-viet-dau-khac-trong-excel/"><img src="https://thuthuatnhanh.com/thuthuatnhanh.jpg" /></a>&plusmn;3</p> <p>Phương tr&igrave;nh được viết lại:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>14</mn><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>&#8722;</mo><mo>&#160;</mo><mn>9</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>&#8201;</mo><mo>+</mo><mo>&#160;</mo><mfrac><mn>1</mn><mrow><mi>x</mi><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>3</mn></mrow></mfrac></math>&nbsp;</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&hArr; 14 = x<sup>2</sup> &minus; 9 + x + 3&nbsp;</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&hArr; x<sup>2</sup> + x &minus; 20 = 0, ∆ = 1+ 4.20 = 81</p> <p>n&ecirc;n x<sub>1&nbsp;</sub>=&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mo>&#8722;</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mn>9</mn></mrow><mn>2</mn></mfrac></math> = &minus;5 ; x<sub>2</sub> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mo>&#8722;</mo><mn>1</mn><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mn>9</mn></mrow><mn>2</mn></mfrac></math> = 4 (thỏa m&atilde;n)</p> <p>Vậy phương tr&igrave;nh c&oacute; hai nghiệm x<sub>1&nbsp;</sub>= &minus;5, x<sub>2&nbsp;</sub>= 4.</p> <p>f)&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mn>2</mn><mi>x</mi></mrow><mrow><mi>x</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mn>1</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>&#160;</mo><mo>&#8722;</mo><mo>&#160;</mo><mi>x</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mn>8</mn></mrow><mrow><mo>(</mo><mi>x</mi><mo>&#160;</mo><mo>+</mo><mn>1</mn><mo>)</mo><mo>(</mo><mi>x</mi><mo>&#8722;</mo><mo>&#160;</mo><mn>4</mn><mo>)</mo></mrow></mfrac></math>. Điều kiện: x &ne;<a title="Nguồn tham khảo từ b&agrave;i viết Dấu kh&aacute;c (&ne;) trong Excel ứng dụng v&agrave; c&aacute;ch viết được thực hiện bởi website Thủ Thuật Nhanh" href="https://thuthuatnhanh.com/cach-viet-dau-khac-trong-excel/"><img src="https://thuthuatnhanh.com/thuthuatnhanh.jpg" /></a> &minus;1, x &ne;<a title="Nguồn tham khảo từ b&agrave;i viết Dấu kh&aacute;c (&ne;) trong Excel ứng dụng v&agrave; c&aacute;ch viết được thực hiện bởi website Thủ Thuật Nhanh" href="https://thuthuatnhanh.com/cach-viet-dau-khac-trong-excel/"><img src="https://thuthuatnhanh.com/thuthuatnhanh.jpg" /></a> 4&nbsp;</p> <p>Phương tr&igrave;nh tương đương với:</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;2x(x &minus; 4) = x<sup>2</sup> &minus; x + 8 &hArr; 2x<sup>2</sup> &minus; 8x &minus; x<sup>2</sup> + x &minus; 8 = 0</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &hArr; x<sup>2</sup> &minus; 7x &minus; 8 = 0&nbsp;</p> <p>C&oacute; a &minus; b + c = 1 &minus; (&minus;7) &minus; 8 = 0 n&ecirc;n x<sub>1</sub> = &minus;1, x<sub>2</sub> = 8</p> <p>V&igrave; x<sub>1</sub> = &minus;1 kh&ocirc;ng thỏa m&atilde;n điều kiện của ẩn n&ecirc;n: phương tr&igrave;nh c&oacute; một nghiệm l&agrave; x = 8.</p>
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