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.

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- Oct 12th 2011, 06:27 PMdazedandmathfusedokay the last verify problem i have problems with
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. - Oct 12th 2011, 08:34 PMProve ItRe: okay the last verify problem i have problems with
$\displaystyle \displaystyle \begin{align*}\frac{1}{2}\left[\cos{(x + y)} + \cos{(x - y)}\right] &\equiv \frac{1}{2}\left[\cos{(x)}\cos{(y)} - \sin{(x)}\sin{(y)} + \cos{(x)}\cos{(y)} + \sin{(x)}\sin{(y)}\right] \\ &\equiv \frac{1}{2}\left[2\cos{(x)}\cos{(y)}\right] \\ &\equiv \cos{(x)}\cos{(y)} \end{align*}$