Find: $\displaystyle cos (\frac{2\pi }{15}) + cos (\frac{4\pi }{15}) +...+ cos (\frac{14\pi }{15})$
Last edited by mr fantastic; Feb 26th 2011 at 12:58 PM. Reason: Re-titled.
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Originally Posted by arm Find: $\displaystyle cos (\frac{2\pi }{15}) + cos (\frac{4\pi }{15}) +...+ cos (\frac{14\pi }{15})$ To get help you need to show some effort. Show your work and say where you're stuck.
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