Đáp án:
$x^{3}$ - 19x + 30 = 0
⇔ $x^{3}$ + 3$x^{2}$ - 10x - 3$x^{2}$ - 9x + 30 = 0
⇔ ( $x^{3}$ - 3$x^{2}$ ) + ( 3$x^{2}$ - 9x ) - ( 10x - 30 ) = 0
⇔ $x^{2}$ ( x - 3 ) + 3x ( x - 3 ) - 10 ( x - 3 ) = 0
⇔ ( $x^{2}$ + 3x - 10 ) ( x - 3 ) = 0
⇔ ( $x^{2}$ - 2x + 5x - 10 ) ( x - 3 ) = 0
⇔ [ x ( x - 2 ) + 5 ( x - 2 ) ] ( x - 3 ) = 0
⇔ ( x + 5 ) ( x - 2 ) ( x - 3 ) = 0
Suy ra x = -5 hoặc x = 2 hoặc x = 3
