Giải câu 55 bài 9: Luyện tập sgk Toán đại 8 tập 1 Trang 25

a) x³ – \(\frac{1}{4}\)x = 0

<=> x(x² –\(\left ( \frac{1}{2} \right )^{2}\)) = 0

<=>x(x - \(\frac{1}{2}\))(x + \(\frac{1}{2}\)) = 0

$\Leftrightarrow \left[ \begin{array}{ll} x = 0 \\ x + \frac{1}{2}=0 \\x - \frac{1}{2}= 0\\\end{array} \right.$

$\Leftrightarrow \left[ \begin{array}{ll} x = 0 \\ x =-\frac{1}{2} \\x = \frac{1}{2}\\\end{array} \right.$

Vậy x = 0; x = -\(\frac{1}{2}\); x = \(\frac{1}{2}\).

b) (2x – 1)² – (x + 3)² = 0

<=>[(2x - 1) - (x + 3)][(2x - 1) + (x + 3)] = 0

<=>(2x - 1 - x - 3)(2x - 1 + x + 3) = 0

<=>(x - 4)(3x + 2) = 0

$\Leftrightarrow \left[ \begin{array}{ll} x - 4 = 0 \\ 3x + 2 =0 \\\end{array} \right.$

$\Leftrightarrow \left[ \begin{array}{ll} x = 4 \\ x =-\frac{2}{3} \\\end{array} \right.$

Vậy x = 4; x = -\(\frac{2}{3}\).

c) x²(x – 3) + 12 – 4x = 0

<=> x²(x – 3) - 4(x -3)= 0

<=> (x - 3)(x²- 2²) = 0

<=> (x - 3)(x - 2)(x + 2) = 0

$\Leftrightarrow \left[ \begin{array}{ll} x - 3 = 0 \\ x + 2 =0 \\x - 2 = 0 \\\end{array} \right.$

$\Leftrightarrow \left[ \begin{array}{ll} x = 3 \\ x =-2 \\x = 2 \\\end{array} \right.$

Vậy x = 3; x = 2; x = -2.