`a)`
`x^3 + 3x^2 + 2x = 0`
`⇔ x^3 + x^2 + 2x^2 + 2x = 0`
`⇔ x^2(x + 1) + 2x(x + 1) = 0`
`⇔ (x^2 + 2x)(x + 1) = 0`
`⇔ x(x + 1)(x + 2) = 0`
`⇒ x = 0 ; -1 ; -2`
`Vậy S = {0;-1;-2}`
`b)`
`(x + 2)^2 - 9(2x - 1)^2 = 0`
`⇔ (x + 2)^2 - [3(2x - 1)]^2 = 0`
`⇔ (x + 2)^2 - (6x - 3)^2 = 0`
`⇔ (x + 2 + 6x - 3)(x + 2 - 6x + 3) = 0`
`⇔ (7x - 1)(-5x + 5) = 0`
`⇒` \(\left[ \begin{array}{l}x = 1/7\\x = 1\end{array} \right.\)
`Vậy S = {1/7 ; 1}`
`c)`
`x^3 + 6x^2 + 11x = 0`
`⇔ x(x^2 + 6x + 11) = 0`
`⇒ x = 0`
`(Do x^2 + 6 + 11 = 0` `text(vô nghiệm))`
`Vậy x = 0`
