Giải các phương trình :
$a) 27 \cos^4x+8\sin x=12\\b)\cos \frac{x}{2}-\cos x +\cos\frac{3x}{2}=\frac{1}{2}\\c) \sin^3x+\cos^3x+\tan^3x =\left ( \sin x +\cos x +\tan x \right )^3\\d)\cos x -3\sqrt{3}\sin x =\cos 7x\\ e) \tan^4 x+\tan^4 2x+\cot ^43x =\frac{1}{3}\\f)\sin^3x+4\cos^3x=3\cos x\\k)\sqrt[4]{4\sin^2x-4\sin x+2}+\sqrt{8\sin^2x-8\sin x +11}=1-12\sin^2x+12\sin x$
hơi nhiều ~~~