$\sin 2x \cos x +\cos 2x \cos x + 2\cos 2x -\sin x = 0$
$\Leftrightarrow \sin 3x +\sin x + \cos 3x +\cos x + 4\cos 2x -2\sin x=0$
$\Leftrightarrow 4(\cos^3 x -\sin^3 x) -2(\cos x -\sin x) +4(\cos x-\sin x)(\sin x +\cos x) = 0$
$\Leftrightarrow (\cos x -\sin x) [4(1+\sin x \cos x) + 4(\sin x + \cos x) -2]=0$
$\Leftrightarrow (\cos x -\sin x) [2\sin 2x +4(\sin x + \cos x) + 2]=0$
$\Leftrightarrow (\cos x -\sin x) [2(1+\sin 2x) +4(\sin x + \cos x) ]=0$
$\Leftrightarrow (\cos x -\sin x) [(\sin x +\cos x)^2 +4(\sin x + \cos x) ]=0$
dễ rồi nhường bạn làm nốt