Câu 1:
$\sin x + 2\cos^2 x \sin x+ \sqrt 3 \cos 3x = 2\cos 4x + 2\sin^3 x$
$\Leftrightarrow \sin x + 2\sin x (\cos^2 x -\sin^2 x) + \sqrt 3 \cos 3x = 2\cos 4x $
$\Leftrightarrow \sin x + 2\sin x \cos 2x + \sqrt 3 \cos 3x = 2\cos 4x $
$\Leftrightarrow \sin x + \sin 3x - \sin x + \sqrt 3 \cos 3x = 2\cos 4x $
Xong