Ôn tập chương IV
Hướng dẫn giải Bài 57 (Trang 63 SGK Toán Đại số 9, Tập 2)
<p>Giải c&aacute;c phương tr&igrave;nh:&nbsp;</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>a</mi><mo>)</mo><mo>&#160;</mo><mn>5</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>3</mn><mi>x</mi><mo>+</mo><mo>=</mo><mn>2</mn><mi>x</mi><mo>+</mo><mn>11</mn><mspace linebreak="newline"/><mi>b</mi><mo>)</mo><mfrac><msup><mi>x</mi><mn>2</mn></msup><mn>5</mn></mfrac><mo>-</mo><mfrac><mrow><mn>2</mn><mi>x</mi></mrow><mn>3</mn></mfrac><mo>=</mo><mfrac><mrow><mi>x</mi><mo>+</mo><mn>5</mn></mrow><mn>6</mn></mfrac><mspace linebreak="newline"/><mi>c</mi><mo>)</mo><mo>&#160;</mo><mfrac><mi>x</mi><mrow><mi>x</mi><mo>-</mo><mn>2</mn></mrow></mfrac><mo>=</mo><mfrac><mrow><mn>10</mn><mo>-</mo><mn>2</mn><mi>x</mi></mrow><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi></mrow></mfrac><mspace linebreak="newline"/><mi>d</mi><mo>)</mo><mo>&#160;</mo><mfrac><mrow><mi>x</mi><mo>+</mo><mn>0</mn><mo>,</mo><mn>5</mn></mrow><mrow><mn>3</mn><mi>x</mi><mo>+</mo><mn>1</mn></mrow></mfrac><mo>=</mo><mfrac><mrow><mn>7</mn><mi>x</mi><mo>+</mo><mn>2</mn></mrow><mrow><mn>9</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>1</mn></mrow></mfrac><mspace linebreak="newline"/><mi>e</mi><mo>)</mo><mo>&#160;</mo><mn>2</mn><msqrt><mn>3</mn></msqrt><msup><mi>x</mi><mn>2</mn></msup><mo>+</mo><mi>x</mi><mo>+</mo><mn>1</mn><mo>=</mo><msqrt><mn>3</mn></msqrt><mfenced><mrow><mi>x</mi><mo>+</mo><mn>1</mn></mrow></mfenced><mspace linebreak="newline"/><mi>f</mi><mo>)</mo><mo>&#160;</mo><msup><mi>x</mi><mn>2</mn></msup><mo>+</mo><mn>2</mn><msqrt><mn>2</mn></msqrt><mi>x</mi><mo>+</mo><mn>4</mn><mo>=</mo><mn>3</mn><mfenced><mrow><mi>x</mi><mo>+</mo><msqrt><mn>2</mn></msqrt></mrow></mfenced></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>5</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>3</mn><mi>x</mi><mo>+</mo><mn>1</mn><mo>=</mo><mn>2</mn><mi>x</mi><mo>+</mo><mn>11</mn><mo>&#8660;</mo><mn>5</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>5</mn><mi>x</mi><mo>-</mo><mn>10</mn><mo>=</mo><mn>0</mn><mo>&#8660;</mo><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mi>x</mi><mo>-</mo><mn>2</mn><mo>=</mo><mn>0</mn></math></p> <p>Phương tr&igrave;nh thỏa m&atilde;n điều kiện <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>a</mi><mo>-</mo><mi>b</mi><mo>+</mo><mi>c</mi><mo>=</mo><mn>1</mn><mo>+</mo><mn>1</mn><mo>-</mo><mn>2</mn><mo>=</mo><mn>0</mn></math> n&ecirc;n c&oacute; hai nghiệm: <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>x</mi><mn>1</mn></msub><mo>=</mo><mo>-</mo><mn>1</mn><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>2</mn></msub><mo>=</mo><mn>2</mn></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>b</mi><mo>)</mo><mfrac><msup><mi>x</mi><mn>2</mn></msup><mn>5</mn></mfrac><mo>-</mo><mfrac><mrow><mn>2</mn><mi>x</mi></mrow><mn>3</mn></mfrac><mo>=</mo><mfrac><mrow><mi>x</mi><mo>+</mo><mn>5</mn></mrow><mn>6</mn></mfrac><mo>&#8660;</mo><mn>6</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>20</mn><mi>x</mi><mo>=</mo><mn>5</mn><mi>x</mi><mo>+</mo><mn>25</mn><mo>&#8660;</mo><mn>6</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>25</mn><mi>x</mi><mo>-</mo><mn>25</mn><mo>=</mo><mn>0</mn><mspace linebreak="newline"/><mo>&#9651;</mo><mo>=</mo><msup><mn>25</mn><mn>2</mn></msup><mo>+</mo><mn>4</mn><mo>.</mo><mn>6</mn><mo>.</mo><mn>25</mn><mo>=</mo><mn>25</mn><mfenced><mrow><mn>25</mn><mo>+</mo><mn>24</mn></mrow></mfenced><mo>=</mo><mn>25</mn><mo>.</mo><mn>49</mn><mo>,</mo><mo>&#160;</mo><msqrt><mo>&#9651;</mo></msqrt><mo>=</mo><mn>35</mn></math></p> <p>Do đ&oacute;:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>x</mi><mn>1</mn></msub><mo>=</mo><mn>5</mn><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>2</mn></msub><mo>=</mo><mo>-</mo><mfrac><mn>5</mn><mn>6</mn></mfrac></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>c</mi><mo>)</mo><mo>&#160;</mo><mfrac><mi>x</mi><mrow><mi>x</mi><mo>-</mo><mn>2</mn></mrow></mfrac><mo>=</mo><mfrac><mrow><mn>10</mn><mo>-</mo><mn>2</mn><mi>x</mi></mrow><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi></mrow></mfrac></math>, Điều kiện: <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi><mo>&#8800;</mo><mn>0</mn><mo>,</mo><mo>&#160;</mo><mi>x</mi><mo>&#8800;</mo><mn>2</mn></math></p> <p>Ta c&oacute;: <math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mi>x</mi><mrow><mi>x</mi><mo>-</mo><mn>2</mn></mrow></mfrac><mo>=</mo><mfrac><mrow><mn>10</mn><mo>-</mo><mn>2</mn><mi>x</mi></mrow><mrow><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>2</mn><mi>x</mi></mrow></mfrac><mo>&#8660;</mo><msup><mi>x</mi><mn>2</mn></msup><mo>=</mo><mn>10</mn><mo>-</mo><mn>2</mn><mi>x</mi><mo>&#8660;</mo><msup><mi>x</mi><mn>2</mn></msup><mo>+</mo><mn>2</mn><mi>x</mi><mo>-</mo><mn>10</mn><mo>=</mo><mn>0</mn></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#9651;</mo><mo>'</mo><mo>=</mo><mn>1</mn><mo>+</mo><mn>10</mn><mo>=</mo><mn>11</mn><mo>&#8658;</mo><msub><mi>x</mi><mn>1</mn></msub><mo>=</mo><mo>-</mo><mn>1</mn><mo>+</mo><msqrt><mn>11</mn></msqrt><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>2</mn></msub><mo>=</mo><mo>-</mo><mn>1</mn><mo>-</mo><msqrt><mn>11</mn></msqrt></math></p> <p>Cả hai gi&aacute; trị n&agrave;y đều thỏa m&atilde;n điều kiện của ẩn. Vậy phương tr&igrave;nh c&oacute; hai nghiệm l&agrave;:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>x</mi><mn>1</mn></msub><mo>=</mo><mo>-</mo><mn>1</mn><mo>+</mo><msqrt><mn>11</mn></msqrt><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>2</mn></msub><mo>=</mo><mo>-</mo><mn>1</mn><mo>-</mo><msqrt><mn>11</mn></msqrt></math>&nbsp;</p> <p>d) Điều kiện <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi><mo>&#8800;</mo><mo>&#177;</mo><mfrac><mn>1</mn><mn>3</mn></mfrac></math></p> <p>Ta c&oacute;:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>x</mi><mo>+</mo><mn>0</mn><mo>,</mo><mn>5</mn></mrow><mrow><mn>3</mn><mi>x</mi><mo>+</mo><mn>1</mn></mrow></mfrac><mo>=</mo><mfrac><mrow><mn>7</mn><mi>x</mi><mo>+</mo><mn>2</mn></mrow><mrow><mn>9</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>1</mn></mrow></mfrac><mo>&#8660;</mo><mfrac><mrow><mn>2</mn><mi>x</mi><mo>+</mo><mn>1</mn></mrow><mrow><mn>3</mn><mi>x</mi><mo>+</mo><mn>1</mn></mrow></mfrac><mo>=</mo><mfrac><mrow><mn>14</mn><mi>x</mi><mo>+</mo><mn>4</mn></mrow><mrow><mn>9</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>1</mn></mrow></mfrac><mspace linebreak="newline"/><mo>&#8660;</mo><mfenced><mrow><mn>2</mn><mi>x</mi><mo>+</mo><mn>1</mn></mrow></mfenced><mfenced><mrow><mn>3</mn><mi>x</mi><mo>-</mo><mn>1</mn></mrow></mfenced><mo>=</mo><mn>14</mn><mi>x</mi><mo>+</mo><mn>4</mn><mspace linebreak="newline"/><mo>&#8660;</mo><mn>6</mn><msup><mi>x</mi><mn>2</mn></msup><mo>+</mo><mi>x</mi><mo>-</mo><mn>1</mn><mo>=</mo><mn>14</mn><mi>x</mi><mo>+</mo><mn>4</mn><mspace linebreak="newline"/><mo>&#8660;</mo><mn>6</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>13</mn><mi>x</mi><mo>-</mo><mn>5</mn><mo>=</mo><mn>0</mn><mspace linebreak="newline"/><mo>&#9651;</mo><mo>=</mo><mn>169</mn><mo>+</mo><mn>120</mn><mo>=</mo><mn>289</mn><mo>,</mo><mo>&#160;</mo><msqrt><mo>&#9651;</mo></msqrt><mo>=</mo><mn>17</mn></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><msub><mi>x</mi><mn>1</mn></msub><mo>=</mo><mfrac><mn>5</mn><mn>2</mn></mfrac><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>2</mn></msub><mo>=</mo><mo>-</mo><mfrac><mn>1</mn><mn>3</mn></mfrac></math>&nbsp; (loại)</p> <p>Vậy phương tr&igrave;nh c&oacute; một nghiệm: <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>x</mi><mn>1</mn></msub><mo>=</mo><mfrac><mn>5</mn><mn>2</mn></mfrac></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>e</mi><mo>)</mo><mo>&#160;</mo><mn>2</mn><msqrt><mn>3</mn></msqrt><msup><mi>x</mi><mn>2</mn></msup><mo>+</mo><mi>x</mi><mo>+</mo><mn>1</mn><mo>=</mo><msqrt><mn>3</mn></msqrt><mfenced><mrow><mi>x</mi><mo>+</mo><mn>1</mn></mrow></mfenced><mo>&#8660;</mo><mn>2</mn><msqrt><mn>3</mn></msqrt><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mfenced><mrow><msqrt><mn>3</mn></msqrt><mo>-</mo><mn>1</mn></mrow></mfenced><mi>x</mi><mo>+</mo><mn>1</mn><mo>-</mo><msqrt><mn>3</mn></msqrt><mo>=</mo><mn>0</mn><mspace linebreak="newline"/><mo>&#9651;</mo><mo>=</mo><msup><mfenced><mrow><msqrt><mn>3</mn></msqrt><mo>-</mo><mn>1</mn></mrow></mfenced><mn>2</mn></msup><mo>=</mo><mn>8</mn><msqrt><mn>3</mn></msqrt><mfenced><mrow><mn>1</mn><mo>-</mo><msqrt><mn>3</mn></msqrt></mrow></mfenced><mo>=</mo><mn>4</mn><mo>-</mo><mn>2</mn><msqrt><mn>3</mn></msqrt><mo>-</mo><mn>8</mn><msqrt><mn>3</mn></msqrt><mo>+</mo><mn>24</mn><mo>=</mo><mn>25</mn><mo>-</mo><mn>2</mn><mo>.</mo><mn>5</mn><msqrt><mn>3</mn></msqrt><mo>+</mo><mn>3</mn><mo>=</mo><msup><mfenced><mrow><mn>5</mn><mo>-</mo><msqrt><mn>3</mn></msqrt></mrow></mfenced><mn>2</mn></msup><mspace linebreak="newline"/><msub><mi>x</mi><mn>1</mn></msub><mo>=</mo><mfrac><mrow><msqrt><mn>3</mn></msqrt><mo>-</mo><mn>1</mn><mo>+</mo><mn>5</mn><mo>-</mo><msqrt><mn>3</mn></msqrt></mrow><mrow><mn>4</mn><msqrt><mn>3</mn></msqrt></mrow></mfrac><mo>=</mo><mfrac><msqrt><mn>3</mn></msqrt><mn>3</mn></mfrac><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>2</mn></msub><mo>=</mo><mfrac><mrow><msqrt><mn>3</mn></msqrt><mo>-</mo><mn>1</mn><mo>-</mo><mn>5</mn><mo>+</mo><msqrt><mn>3</mn></msqrt></mrow><mrow><mn>4</mn><msqrt><mn>3</mn></msqrt></mrow></mfrac><mo>=</mo><mfrac><mrow><mn>1</mn><mo>-</mo><msqrt><mn>3</mn></msqrt></mrow><mn>2</mn></mfrac><mspace linebreak="newline"/><mi>f</mi><mo>)</mo><mo>&#160;</mo><msup><mi>x</mi><mn>2</mn></msup><mo>+</mo><mn>2</mn><msqrt><mn>2</mn></msqrt><mi>x</mi><mo>+</mo><mn>4</mn><mo>=</mo><mn>3</mn><mfenced><mrow><mi>x</mi><mo>+</mo><msqrt><mn>2</mn></msqrt></mrow></mfenced><mo>&#8660;</mo><msup><mi>x</mi><mn>2</mn></msup><mo>+</mo><mfenced><mrow><mn>2</mn><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>3</mn></mrow></mfenced><mo>+</mo><mn>4</mn><mo>-</mo><mn>3</mn><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>0</mn><mspace linebreak="newline"/><mo>&#9651;</mo><mo>=</mo><mn>8</mn><mo>-</mo><mn>12</mn><msqrt><mn>2</mn></msqrt><mo>+</mo><mn>9</mn><mo>-</mo><mn>16</mn><mo>+</mo><mn>12</mn><msqrt><mn>2</mn></msqrt><mo>=</mo><mn>1</mn></math></p> <p>Do đ&oacute;:&nbsp;<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><mn>2</mn><msqrt><mn>2</mn></msqrt><mo>+</mo><mn>1</mn></mrow><mn>2</mn></mfrac><mo>=</mo><mn>2</mn><mo>-</mo><msqrt><mn>2</mn></msqrt><mo>,</mo><mo>&#160;</mo><msub><mi>x</mi><mn>2</mn></msub><mo>=</mo><mfrac><mrow><mn>3</mn><mo>-</mo><mn>2</mn><msqrt><mn>2</mn></msqrt><mo>-</mo><mn>1</mn></mrow><mn>2</mn></mfrac><mo>=</mo><mn>1</mn><mo>-</mo><msqrt><mn>2</mn></msqrt></math></p>
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