$(1 + 2\sin x)^2 \cos x - \cos x = 1 + \sin x$
$\Leftrightarrow \cos x [ (1 + 2\sin x)^2 - 1 ] = 1 + \sin x$
$\Leftrightarrow 2\sin x \cos x (2 + 2\sin x) = 1 + \sin x$
$\Leftrightarrow (1 + \sin x) [ 4\sin x \cos x - 1 ] = 0$
$\Leftrightarrow (1 + \sin x)(2\sin 2x - 1) = 0$
Dễ rồi bạn nhé