$\Leftrightarrow \cos 2x \cos x +2\sin^2 x -\cos^2 x =2\cos x$
$\Leftrightarrow (2\cos^2 x -1)\cos x +2(1-\cos^2 x)-\cos^2 x-2\cos x=0$
$\Leftrightarrow2\cos^3 x -3\cos^2 x-3\cos x +2 = 0$
$\Leftrightarrow 2(\cos^3 x+1) -3\cos x(\cos x +1)=0$
$\Leftrightarrow 2(\cos x+1)(\cos^2 x -\cos x +1) -3\cos x(\cos x +1)=0$
$\Leftrightarrow (\cos x +1)(2\cos^2 x-5\cos x+2)=0$
$\Leftrightarrow (\cos x +1)(\cos x -2)(2\cos x-1)=0$ dễ rồi tự giải nhé