Ta có: $(\cos x; \sin x)=(0;\pm 1)$ không là nghiệm PT
Xét $\cos x\ne 0\to x\ne \dfrac{\pi}{2}+k\pi$
Chia hai vế cho $\cos^3x$:
$1-4\tan^3x-3\tan^2x+\tan x(1+\tan^2x)=0$
$\to -3\tan^3x-3\tan^2x+\tan x+1=0$
$\to \left[ \begin{array}{l}\tan x=-1\\ \tan x=\pm\dfrac{1}{\sqrt3} \end{array} \right.$
$\to \left[ \begin{array}{l}x=\dfrac{-\pi}{4}+k\pi \\x=\pm\dfrac{\pi}{6}+k\pi\end{array} \right.$ (TM)
