Câu 1. $4\cos^3 x+6\sqrt 2\sin x \cos x-8\cos x=0$
$\Leftrightarrow \cos x (2\cos^2 x +3\sqrt 2 \sin x -4)=0$ dễ rồi
Chú ý $ 2\cos^2 x +3\sqrt 2 \sin x -4=0$
$\Leftrightarrow 2(1-\sin^2 x)+3\sqrt 2 \sin x -4=0$ tự làm
Câu 2 Điều kiện $\sin 2x \ne 0 $ tự làm
PT $\Leftrightarrow 2\sin 3x \sin x \cos x-\cos x= 2\cos 3x \sin x \cos x +\sin x$
$\Leftrightarrow \sin 3x \sin 2x -\cos 3x \sin 2x -\sin x -\cos x=0$
$\Leftrightarrow \sin 2x (\sin 3x -\cos 3x) -(\sin x +\cos x)=0$
$\Leftrightarrow \sin 2x (3\sin x -4\sin^3 x -4\cos^3 x +3\cos x)-(\sin x +\cos x)=0$
$\Leftrightarrow \sin 2x \bigg [ 3(\sin x+\cos x) -4(\sin x +\cos x)(1-\sin x \cos x )\bigg ]-(\sin x +\cos x)=0$
+ $\sin x +\cos x = 0$ tự làm
+ $\sin 2x (3-4+4\sin x \cos x) -1=0$
$\Leftrightarrow \sin 2x (-1+2\sin 2x)-1=0$ tự làm