A closed expression for $\sum_{k=1}^n\cos(kA)$

Asked Nov 05 '15 at 17:19

Active Nov 05 '15 at 17:38

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How do I find a closed expression for $$\sum_{k=1}^n\cos(kA)$$ any idea about how to go about doing this?

Thanks in advance

edited Nov 05 '15 at 17:38

asked Nov 05 '15 at 17:19

user283172

2

Note that your expression is the real part of $\sum_{k=1}^{n}\exp(ikA)$, which can be evaluated in closed form as it is the partial sum of a geometric series. – Nov 05 '15 at 17:20

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