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Possible Duplicate:
How can we sum up $\sin$ and $\cos$ series when the angles are in arithmetic progression?

Prove $$\cos(\alpha) + \cos(\alpha + \beta) + \cos(\alpha + 2\beta) + \dots + \cos[\alpha + (n-1)\beta] = \frac{\cos(\alpha + \frac{n-1}{2}\beta) \cdot \sin\frac{n\beta}{2}}{\sin\frac{\beta}{2}} $$

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    You first! (Please don't write your question in the imperative. If it's *your* assignment to prove the identity, please let us know what you've already tried.)2012-03-06

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