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)(x2 + 1) = 0
nếu số x2 là 2.x thì
\(\left[ \begin{array}{l}x-1=0\\2x+1=0\end{array} \right.\) (=)\(\left[ \begin{array}{l}x=1\\2x=-1\end{array} \right.\) (=)\(\left[ \begin{array}{l}x=1\\x=\frac{-1}{2}\end{array} \right.\)
nếu số x2 là x² thì
\(\left[ \begin{array}{l}x-1=0\\x^{2}+1=0\end{array} \right.\) (=)\(\left[ \begin{array}{l}x=1\\x^{2}=-1=) vô lý\end{array} \right.\) =)x=1
