`#Sad`
`1)`
`x^3+4x^2+4x = 0`
`⇔ x(x^2+4x+4) = 0`
`⇔ x(x+2)^2 = 0`
`⇔` \(\left[ \begin{array}{l}x=0\\x+2=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=0\\x=-2\end{array} \right.\)
`\text{Vậy S=}` `{0; -2}`
`2)`
`(x+3)^2-4 = 0`
`⇔ (x+3-2)(x+3+2) = 0`
`⇔ (x+1)(x+5) = 0`
`⇔` \(\left[ \begin{array}{l}x+1=0\\x+5=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=-1\\x=-5\end{array} \right.\)
`\text{Vậy S=}` `{-1; -5}`
`3)`
`x^4-9x^2 = 0`
`⇔ x^2(x^2-9) = 0`
`⇔ x^2(x-3)(x+3) = 0`
`⇔` \(\left[ \begin{array}{l}x^2=0\\x-3=0\\x+3=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=0\\x=3\\x=-3\end{array} \right.\)
`\text{Vậy S=}` `{0; 3; -3}`
`4)`
`x^2-6x+9 = 81`
`⇔ (x-3)^2 = 81`
`⇔ (x-3)^2 = (+-9)^2`
`⇔` \(\left[ \begin{array}{l}x-3=9\\x-3=-9\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=12\\x=-6\end{array} \right.\)
`\text{Vậy S=}` `{12; -6}`
`5)`
`x^3+6x^2+9x-4x = 0`
`⇔ x^3+6x^2+5x = 0`
`⇔ x(x^2+6x+9)-4 = 0`
`⇔ x(x+3)^2-2^2 = 0`
`⇔ x(x+3-2)(x+3+2) = 0`
`⇔ x(x+1)(x+5) = 0`
`⇔` \(\left[ \begin{array}{l}x=0\\x+1=0\\x+5=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=0\\x=-1\\x=-5\end{array} \right.\)
`\text{Vậy S=}` `{0; -1; -5}`
