a/ $x(x+3)=0$
\(\leftrightarrow\left[ \begin{array}{l}x=0\\x+3=0\end{array} \right.\) \(\leftrightarrow\left[ \begin{array}{l}x=0\\x=-3\end{array} \right.\)
b/ $(x-2)(5-x)=0$
\(\leftrightarrow\left[ \begin{array}{l}x-2=0\\5-x=0\end{array} \right.\) \(\leftrightarrow\left[ \begin{array}{l}x=2\\x=5\end{array} \right.\)
c/ $(x-1)(x²+1)=0$
$→x-1=0$ (vì $x²+1>0$)
$↔x=1$