Tìm x biết x² – 9 + 5(x – 3) = 0.

Ta có:

x² – 9 + 5(x – 3) = 0

⇔ x² – 9 + 5x – 15 = 0

⇔ x² + 5x – 24 = 0

⇔ x² – 3x + 8x – 24 = 0

⇔ x(x – 3) + 8(x – 3) = 0

⇔ (x – 3)(x + 8) = 0

\( \Leftrightarrow \left[ \begin{array}{l}x - 3 = 0\\x + 8 = 0\end{array} \right.\)\( \Leftrightarrow \left[ \begin{array}{l}x = 3\\x = - 8\end{array} \right.\)

Vậy x = 3 hoặc x = –8.