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Math Help - please solve

  1. #1
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    please solve

    Find the sum to n terms of the series Cos^2 a + Cos^2 2a+Cos^2 3a+………..
    Last edited by whackywheelz; September 1st 2007 at 02:12 AM. Reason: its raised to 2
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by whackywheelz View Post
    Find the sum to n terms of the series Cos^2 a + Cos^2 2a+Cos^2 3a+..
    <br />
\sum_{r=1}^n \cos^2(r \alpha) =\sum_{r=1}^n (1/2)[1-\cos(2r \alpha)] = n/2 -(1/2) \sum_{r=1}^n \cos(2r \alpha) <br />

    and I presume you can find \sum_{r=1}^n \cos(2r \alpha) yourself

    RonL
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  3. #3
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    please solve upto final answer

    Hi,

    I've had a gap of ten years on the subject, so can please solve up to the final answer.

    Thanks
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  4. #4
    Grand Panjandrum
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    Quote Originally Posted by whackywheelz View Post
    Hi,

    I've had a gap of ten years on the subject, so can please solve up to the final answer.

    Thanks
    Do you know about complex numbers? Particularly the relationship between cartesian and
    polar forms (or DeMoivre's theorem)? As I will need these for the next bit if I do this the
    neatest way.

    RonL
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