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Math Help - okay the last verify problem i have problems with

  1. #1
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    okay 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.
    Last edited by dazedandmathfused; October 12th 2011 at 08:01 PM.
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  2. #2
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    Re: okay the last verify problem i have problems with

    Quote Originally Posted by dazedandmathfused View Post
    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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