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

Got it !

## Solve:

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