a) $x.(x + y) = 0 ⇔ $ \(\left[ \begin{array}{l}x=0\\x + y= 0\end{array} \right. ⇔ \left[ \begin{array}{l}x=0\\y = 0\end{array} \right.\)
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) $(x - 1).(x^{2} + 1) = 0 ⇔ x - 1 = 0 ⇔ x = 1$ (vì $x^{2} + 1 > 0$ với mọi $x$)