Solve by: $ab=0 \rightarrow $ : $a=0 $ ou $b=0$:

Got it !

Solve:

$\LARGE x^2-9 = 0$	$\LARGE (x+3)(x-3) = 0 $ so: $\LARGE S=\{-3;3\}$
$\LARGE x^2-3x=0 $	$\LARGE S=\{0;3\}$
$\LARGE x^2+10x+21=0 $	$\LARGE S=\{-3;-7\}$
$\LARGE x^2+2x=-1 $	$\LARGE (x+1)^2=0 ; S=\{-1\} $
$\LARGE (x-9)^2=1 $	$\LARGE (x-10)(x-8)=0 ; S=\{10;8\}$
$\LARGE x^3+x^2=4x+4 $	$\LARGE S=\{-1;-2;2\}$
$\LARGE x^5=2x^4+x-2 $	$\LARGE (x^4 - 1)(x - 2) = 0 \;\; ; \;\; S=\{-1;1;2\}$