Đk : $\cos x\neq 0; \cos 2x\neq 0; \cos 3x \neq 0$PT đã cho
$\Leftrightarrow \cos 3x.\sin^2x+\cos x.sin^23x=\sin 2x.2\sin x$
$\Leftrightarrow \cos x.\sin^2x(4\cos^2x-3)+\cos x.\sin^2x(3-4\sin^2x)^2=4\sin^2x.\cos x$
$\Leftrightarrow \cos x.\sin^2x[(4\cos^2x-1)^2+(4\cos^2x-1)-6]=0$ (1)
Đặt $4cos^2x-1=t$
(1)$\Leftrightarrow \cos x.\sin^2x.(t^2+t-6)=0$