Find: $\displaystyle cos ^{2}(\frac{\pi }{5})+ cos^{2} (\frac{2\pi }{15})+ cos^{2} (\frac{8\pi }{15})$

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- Feb 26th 2011, 10:25 AMarmSum of squares of cosine.
Find: $\displaystyle cos ^{2}(\frac{\pi }{5})+ cos^{2} (\frac{2\pi }{15})+ cos^{2} (\frac{8\pi }{15})$

- Feb 26th 2011, 12:56 PMmr fantastic