2sin(x)²+sin(x)cos(x) -3cos(x)²=0 (1)
_ Xét sin(x)=0 ⇒ sin(x)²=0 thay vào (1) ⇔ 3cos(x)²=0
⇔ cos(x)²=0 (vô lí ) ( vì sin(x)²+cos(x)²=1)
⇒ sin(x)$\neq$ 0
(1)⇔ 2+tan(x)-3tan(x)²=0 ( chia cả 2 vế cho sin(x)² )
⇔ \(\left[ \begin{array}{l}tan(x)=1\\tan(x)=\frac{-2}{3}\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=\frac{\pi}{4}+k\pi\\x=acr tan(\frac{-2}{3})+k\pi\end{array} \right.\)
