ĐK : \sin 2x \neq 0
PT \Leftrightarrow 2\frac{\sin x}{\cos x} + \frac{\cos x}{\sin x} - 2\sin x\cos x - \frac{1}{\sin x}
\Leftrightarrow \sin x^{2} + \sin x^{2} + 1 + \cos x^{2} - 4\sin x^{2}\cos x^{2} - \cos x = 0
\Leftrightarrow 2 - \cos x^{2} - 4\sin x^{2}\cos x - \cos x = 0
\Leftrightarrow 2 - \cos x^{2} - 4(1 - \cos x^{2} )\cos x - \cos x = 0
\Leftrightarrow 4\cos x^{3} - \cos x^{2} - 5\cos x + 2 = 0
\Leftrightarrow \cos x = 1 \vee \cos x = \frac{-3+\sqrt{41}}{8}