Tìm x, biếta) x(x – 6) – x2= 18;b) 25x2– 64 = 0.

Hướng dẫn giải

a) x(x – 6) – x²= 18

⇔ x²– 6x – x²= 18

⇔ – 6x = 18

⇔ x = – 3

Vậy x = –3.

b) 25x²– 64 = 0

⇔ 25x²– 8²= 0

⇔ (5x – 8)(5x + 8) = 0

\(\begin{array}{l} \Leftrightarrow \left[ \begin{array}{l}5x - 8 = 0\\5x + 8 = 0\end{array} \right.\\ \Leftrightarrow \left[ \begin{array}{l}x = \frac{8}{5}\\x = - \frac{8}{5}\end{array} \right.\end{array}\)

Vậy \(x = \pm \frac{8}{5}\).