Hướng dẫn giải Bài 13 (Trang 9 SGK Toán Đại số 8, Tập 1)

Tìm x, biết: <math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced><mrow><mn>12</mn><mi>x</mi><mo>-</mo><mn>5</mn></mrow></mfenced><mfenced><mrow><mn>4</mn><mi>x</mi><mo>-</mo><mn>1</mn></mrow></mfenced><mo>+</mo><mfenced><mrow><mn>3</mn><mi>x</mi><mo>-</mo><mn>7</mn></mrow></mfenced><mfenced><mrow><mn>1</mn><mo>-</mo><mn>16</mn><mi>x</mi></mrow></mfenced><mo>=</mo><mn>81</mn></math> Giải: <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle indentalign="center"><mfenced><mrow><mn>12</mn><mi>x</mi><mo>-</mo><mn>5</mn></mrow></mfenced><mfenced><mrow><mn>4</mn><mi>x</mi><mo>-</mo><mn>1</mn></mrow></mfenced><mo>+</mo><mfenced><mrow><mn>3</mn><mi>x</mi><mo>-</mo><mn>7</mn></mrow></mfenced><mfenced><mrow><mn>1</mn><mo>-</mo><mn>16</mn><mi>x</mi></mrow></mfenced><mo>=</mo><mn>81</mn><mspace linebreak="newline"/><mn>48</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>12</mn><mi>x</mi><mo>-</mo><mn>20</mn><mi>x</mi><mo>+</mo><mn>5</mn><mo>+</mo><mn>3</mn><mi>x</mi><mo>-</mo><mn>48</mn><msup><mi>x</mi><mn>2</mn></msup><mo>-</mo><mn>7</mn><mo>+</mo><mn>112</mn><mi>x</mi><mo>=</mo><mn>81</mn><mspace linebreak="newline"/><mn>83</mn><mi>x</mi><mo>-</mo><mn>2</mn><mo>=</mo><mn>81</mn><mspace linebreak="newline"/><mn>83</mn><mi>x</mi><mo>=</mo><mn>83</mn><mspace linebreak="newline"/><mi>x</mi><mo>=</mo><mn>1</mn><mspace linebreak="newline"/></mstyle></math>