$\sin 5x -\sin x = 4\sin x$
$\Leftrightarrow 2\cos 3x \sin 2x =4\sin x$
$\Leftrightarrow 4\sin x ( \cos 3x \cos x-1)=0$
+ $\sin x =0$ xong nhé
+ $ \cos 3x \cos x-1=0$
$\Leftrightarrow \cos 4x + \cos 2x =2$
$\Leftrightarrow 2\cos^2 2x + \cos 2x -3=0$ Dễ rồi nhé