Chứng minh:
a) $\cos(x+y) \sin(x-y)+\cos(y+z) \sin(y-z)+ \cos(z+x) \sin(z-x) =0$
b) $\sin x. \sin(y-z) +\sin y. \sin(z-x) +\sin z. \sin (x-y) =0$
c) $\cos (x+y+z)+\cos(x+y-z)+\cos(y+z-x)+\cos (x+z-y)$$ = 4\cos x \cos y \cos z$
d) $1+2\cos 2x+ 2\cos 4x+ 2\cos 6x+ 2\cos 8x =\frac{\sin 9x}{\sin x} $
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