Ta có
$(x+1)(2-x) - (5x+5)(x+2) = -4x^2 + 2$
$\Leftrightarrow 2x - x^2 + 2 - x - (5x^2 + 10x + 5x + 10) = -4x^2 + 2$
$\Leftrightarrow 2x - x^2 + 2 - x - 5x^2 - 10x - 5x - 10 = -4x^2 + 2$
$\Leftrightarrow -2x^2 -14x -10 = 0$
$\Leftrightarrow x^2 + 7x + 10 = 0$
$\Leftrightarrow (x+2)(x+5) = 0$
Vậy $x = -2$ hoặc $x = -5$
Vậy $S = \{-5, -2\}$.