Prove:$\displaystyle cos (\frac{\pi }{7})- cos (\frac{2\pi }{7})+ cos (\frac{3\pi }{7})=\frac{1}{2}$

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- Feb 26th 2011, 10:27 AMarmSum of cosines.
Prove:$\displaystyle cos (\frac{\pi }{7})- cos (\frac{2\pi }{7})+ cos (\frac{3\pi }{7})=\frac{1}{2}$

- Feb 26th 2011, 12:53 PMmr fantastic