Prove:$\displaystyle cos (\frac{\pi }{7})- cos (\frac{2\pi }{7})+ cos (\frac{3\pi }{7})=\frac{1}{2}$
Last edited by mr fantastic; Feb 26th 2011 at 12:54 PM. Reason: Re-titled.
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Originally Posted by arm Prove:$\displaystyle cos (\frac{\pi }{7})- cos (\frac{2\pi }{7})+ cos (\frac{3\pi }{7})=\frac{1}{2}$ Please make some effort. Show your work and say where you're stuck.
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