$(x+1)(x+4)=(2-x)(2+x)$
$⇔ x² +5x +4 = 4 -x²$
$⇔ x² +x² +5x = 4 -4$
$⇔ 2x² +5x = 0$
$⇔ x.(2x +5) = 0 $
$⇔ \(\left[ \begin{array}{l}x=0\\2x +5=0\end{array} \right.\) $
$⇔ \(\left[ \begin{array}{l}x=0\\x=\frac{-5}{2}\end{array} \right.\) $
Vậy S= { 0; $\frac{-5}{2}$ }