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Math Help - Sum of cosines.

  1. #1
    arm
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    Sum of cosines.

    Prove:  cos (\frac{\pi }{7})- cos (\frac{2\pi }{7})+ cos (\frac{3\pi }{7})=\frac{1}{2}
    Last edited by mr fantastic; February 26th 2011 at 01:54 PM. Reason: Re-titled.
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  2. #2
    Flow Master
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    Quote Originally Posted by arm View Post
    Prove:  cos (\frac{\pi }{7})- cos (\frac{2\pi }{7})+ cos (\frac{3\pi }{7})=\frac{1}{2}
    Please make some effort. Show your work and say where you're stuck.
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