$x(x+3)=0$
$⇔$ \(\left[ \begin{array}{l}x=0\\x+3=0\end{array} \right.\)
$⇔$ \(\left[ \begin{array}{l}x=0\\x=-3\end{array} \right.\)
$Vậy...$
$--$
$(x-2)(5-x)=0$
$⇔$ \(\left[ \begin{array}{l}x-2=0\\5-x=0\end{array} \right.\)
$⇔$ \(\left[ \begin{array}{l}x=2\\x=5\end{array} \right.\)
$Vậy...$
$--$
$(x-1)(x^2+1)=0$
$⇔$ \(\left[ \begin{array}{l}x-1=0\\x^2+1=0\end{array} \right.\)
$⇔$ \(\left[ \begin{array}{l}x=1\\x^2=-1(loại)\end{array} \right.\)
$Vậy...$