$3\sin^4 x+ 2\cos^2 3x+\cos 3x=3\cos^4 x - \cos x+1$
$\Leftrightarrow 3(\sin^4 x- \cos^4 x) + 2\cos^2 3x+\cos 3x +\cos x - 1 = 0$
$\Leftrightarrow 3(\sin^2 x - \cos^2 x) + 2\cos^2 3x+\cos 3x +\cos x - 1 = 0$
$\Leftrightarrow \cos 6x - 3\cos 2x + \cos 3x +\cos x = 0$
$\Leftrightarrow (\cos 6x + \cos 2x) +(\cos 3x +\cos x) - 4\cos 2x = 0$
$\Leftrightarrow 2\cos 4x \cos 2x + 2\cos 2x \cos x - 4\cos 2x = 0$
$\Leftrightarrow \cos 2x(\cos 4x + \cos x - 2 ) = 0$
$\Leftrightarrow \left [ \begin{matrix} \cos 2x = 0 \\ \begin{cases} \cos 4x = 1 \\ \cos x = 1 \end{cases} \end{matrix} \right.$
$\Leftrightarrow \left [ \begin{matrix} \cos 2x = 0 \\ \cos x = 1 \end{matrix} \right.$ dễ rồi tự làm