a)
$x.(x+3)=0$
⇔\(\left[ \begin{array}{l}x=0\\x+3=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=0\\x=-3\end{array} \right.\)
b)
$(x-2)(5-x) =0$
⇔\(\left[ \begin{array}{l}x-2=0\\5-x=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=2\\x=5\end{array} \right.\)
c)
$(x+1)(x^{2} +1)=0$
⇔\(\left[ \begin{array}{l}x+1=0\\x^{2} +1=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=-1\\x^{2}=-1\text{(vô nghiệm)}\end{array} \right.\)
