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I am confused on how to verify an identity with three variables.

Problem: $$\cos(x+y+z) = \cos(x) \cos(y) \cos(z)$$

Where do I start? Thanks

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Take $x=0,y=\pi/2,z=\pi/2$ and you find $0=-1$. Conclude. – user63181 Mar 14 '13 at 19:47

Try setting $z= -y$, this would give $\cos(x) = \cos(x) \cos(y)^2$.

Setting $x=0$ would give $1 = \cos(y)^2$. Can this be true for all $y$?

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