$(x²-x)² -2 = x² -x$
$⇔ x^{4} -2x³ +x² -2 = x² -x$
$⇔ x^{4} -2x³ +x² -x² -2 +x = 0$
$⇔ x^{4} -2x³ +x -2 = 0$
$⇔ x³.(x -2) + (x -2) = 0$
$⇔ (x -2).(x³ +1) = 0$
$⇔ \left[ \begin{array}{l}x -2=0\\x³ +1=0\end{array} \right. ⇔ \left[ \begin{array}{l}x=2\\x=-1\end{array} \right.$
$Vậy$ $S =$ {$2; -1$}