# Math Help - please solve

Find the sum to n terms of the series Cos^2 a + Cos^2 2a+Cos^2 3a+………..

2. Originally Posted by whackywheelz
Find the sum to n terms of the series Cos^2 a + Cos^2 2a+Cos^2 3a+………..
$
\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)
$

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

RonL

Hi,

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

Thanks

4. Originally Posted by whackywheelz
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