`2)`
`a)`
`6x^3 - 24x = 0`
`⇔ 6x(x^2 - 4) = 0`
`⇒` \(\left[ \begin{array}{l}x = 0\\x=±2\end{array} \right.\)
`Vậy S = {-2 ; 0 ; 2}`
`b)`
`x(x - 3) + 12 - 4x = 0`
`⇔ x(x - 3) - 4(x - 3) = 0`
`⇔ (x - 3)(x - 4) = 0`
`⇒` \(\left[ \begin{array}{l}x = 3\\x=4\end{array} \right.\)
`Vậy S = {3 ; 4}`
`c)`
`x^3 - 16x = 0`
`⇔ x(x^2 - 16) = 0`
`⇒` \(\left[ \begin{array}{l}x = 0\\x=±4\end{array} \right.\)
`Vậy S = {-4 ; 0 ; 4}`
`3)`
`A = 15 - 2x^2 - 3x`
`A = -2(x^2 + 3/2x - 15/2)`
`A = -2(x^2 + 2 . x . 3/4 + 9/16 - 129/16)`
`A = -2(x + 3/4)^2 + 129/8 ≤ 129/8`
`text(Max)` `A = 129/8` `text(khi)` `x = -3/4`
