Đk : $\sin 4x\neq 0$
Pt đã cho $\Leftrightarrow \frac{-\cos 2x}{\frac{1}{2}\sin 2x}+\frac{2\sin 2x}{cos 2x}+\cot^3 4x=3$$\Leftrightarrow \frac{2\sin^22x-2\cos^22x}{\sin 2x\cos 2x}+\cot^34x=3$
$\Leftrightarrow \frac{-4\cos 4x}{\sin 4x}+\cot^34x=3$
$\Leftrightarrow \cot^34x-4\cot 4x-3=0$
$\Leftrightarrow (\cot 4x+1)(\cot^24x-\cot 4x-3)=0$