okay the last verify problem i have problems with

**dazedandmathfused** · October 12th 2011, 06:27 PM

Verify cosxcosy = 1/2 [cos(x+y) + cos(x-y)]

I think the inner part is goofing me up.

I never get far . Please help.

**Prove It** · October 12th 2011, 08:34 PM

Originally Posted by dazedandmathfused

Verify cosxcosy = 1/2 [cos(x+y) + cos(x-y)]

I think the inner part is goofing me up.

I never get far . Please help.

$\displaystyle \begin{align*}\frac{1}{2}\left[\cos{(x + y)} + \cos{(x - y)}\right] &\equiv \frac{1}{2}\left[\cos{(x)}\cos{(y)} - \sin{(x)}\sin{(y)} + \cos{(x)}\cos{(y)} + \sin{(x)}\sin{(y)}\right] \\ &\equiv \frac{1}{2}\left[2\cos{(x)}\cos{(y)}\right] \\ &\equiv \cos{(x)}\cos{(y)} \end{align*}$

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