Hi!

$\displaystyle \sum_{k=3}^{\infty} ln(cos(\frac{\pi}{k})) $ , convergent or divergent?

I tried:

$\displaystyle a_{k}=ln(1+(cos(\frac{\pi}{k}) -1 ) $

Now using the Taylor expansion of $\displaystyle ln(1+x) = x-\frac{x^2}{2}+...$ with $\displaystyle x=(cos(\frac{\pi}{k})-1) $ AND using the taylor expansion of $\displaystyle cos(x)=1-\frac{x^2}{2}+... $ I got

$\displaystyle a_{k}=\frac{-\pi^{2}}{2k^{2}}+O(\frac{1}{k^{4}}) $

To be able to use the comparison critera, $\displaystyle \lim_{k\to \infty} \frac{a_{k}}{b_{k}} = A > 0 $ , I applied it with $\displaystyle b_{k}=\frac{1}{k^{2}} $ on $\displaystyle - \sum_{k=3}^{\infty} ln(cos(\frac{\pi}{k})) $

And hence $\displaystyle a_{k}=\frac{\pi^{2}}{2k^{2}}-O(\frac{1}{k^{4}}) $

And $\displaystyle \lim_{k\to \infty} \frac{a_{k}}{b_{k}} = \frac{\pi^2}{2}>0 $

And $\displaystyle \sum_{k=1}^{\infty} \frac{1}{k^2} $ converges.

So the original series converges.

I assume if $\displaystyle \sum_{k=3}^{\infty} ln(cos(\frac{\pi}{k})) =s $ then $\displaystyle - \sum_{k=3}^{\infty} ln(cos(\frac{\pi}{k})) =-s $ ?

Basically what I want, is someone to check the work.

Thx!