`x^3 = x`
`⇔ x^3 - x = 0`
`⇔ x^2 . x - x = 0`
`⇔ x . ( x^2 - 1 ) = 0`
`⇔` $\left[\begin{matrix}x = 0\\x^2 - 1 = 0\end{matrix}\right.$
`⇔` $\left[\begin{matrix}x = 0\\x^2 = 1\end{matrix}\right.$
`⇔` $\left[\begin{matrix}x = 0\\x = 1\\ x = - 1\end{matrix}\right.$
Vậy `, x ∈ { 0 ; 1 ; - 1 }`