`x^8 = x`
`=> x^8 - x = 0`
`=> x . x^7 - x = 0`
`=> x . (x^7 - 1) = 0`
`=>` \(\left[ \begin{array}{l}x=0\\x^7 - 1 = 0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x = 0\\x^7 = 1\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x=0\\x=1\end{array} \right.\)
Vậy `x ∈ {0 ; 1}`