a) x³ - x = 0
x(x² - 1) = 0
x(x - 1)(x + 1) = 0
x = 0 hoặc x - 1 = 0 hoặc x + 1 = 0
*) x - 1 = 0
x = 0 + 1
x = 1
*) x + 1 = 0
x = 0 - 1
x = -1
Vậy x = -1; x = 0; x = 1
b) x³ - 2x² - 9x + 18 = 0
(x³ - 2x²) + (-9x + 18) = 0
x²(x - 2) - 9(x - 2) = 0
(x - 2)(x² - 9) = 0
(x - 2)(x - 3)(x + 3) = 0
x - 2 = 0 hoặc x - 3 = 0 hoặc x + 3 = 0
*) x - 2 = 0
x = 0 + 2
x = 2
*) x - 3 = 0
x = 0 + 3
x = 3
*) x + 3 = 0
x = 0 - 3
x = -3
Vậy x = -3; x = 2; x = 3
c) (x² + 4)² - 16x² = 0
(x² + 4 - 4x)(x² + 4 + 4x) = 0
(x - 2)²(x + 2)² = 0
(x - 2)² = 0 hoặc (x + 2)² = 0
*) (x - 2)² = 0
x - 2 = 0
x = 0 + 2
x = 2
*) (x + 2)² = 0
x + 2 = 0
x = 0 - 2
x = -2
Vậy x = -2; x = 2
