x(x+3)=0
(=)\(\left[ \begin{array}{l}x=0\\x+3=0\end{array} \right.\)(=)\(\left[ \begin{array}{l}x=0\\x=-3\end{array} \right.\)
(x-2)(5-x)=0
(=)\(\left[ \begin{array}{l}x-2=0\\5-x=0\end{array} \right.\)(=)\(\left[ \begin{array}{l}x=2\\x=5\end{array} \right.\)
(x-1)(x ^2 +1)=0
(=)\(\left[ \begin{array}{l}x-1=0\\x^{2}+1=0\end{array} \right.\) (=)\(\left[ \begin{array}{l}x=1\\x^{2}=-1=)vô lý \end{array} \right.\)