\(x^3+x=0\)
\(\Rightarrow x\left(x^2+1\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}x=0\\x^2+1=0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=0\\x^2=-1\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=0\\x\in\varnothing\end{matrix}\right.\)