Tìm x biết: x2 + x = 0

x² + x = 0

⇔ x(x + 1) = 0

⇔ \(\left[ \begin{array}{l}x = 0\\x + 1 = 0\end{array} \right.\)

⇔ \(\left[ \begin{array}{l}x = 0\\x = - 1\end{array} \right.\)

Vậy x = 0 hoặc x = -1.