\(x^4+4x^3+4x^2=0\\ \Leftrightarrow x^2\left(x^2+4x+4\right)=0\\ \Leftrightarrow x^2\left(x^2+2.x.2+2^2\right)=0\\ \Leftrightarrow x^2\left(x+2\right)^2=0\\ \Leftrightarrow\left[{}\begin{matrix}x^2=0\\\left(x+2\right)^2=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=0\\x=-2\end{matrix}\right.\)
Vậy \(x\in\left\{0;-2\right\}\)