Parseval - how to go from complex to real coefficients?

1. Oct 4, 2011

Inertigratus

1. The problem statement, all variables and given/known data
I'd like to use Parseval's equation to somehow express $\sum a^{2}_{k}$

2. Relevant equations
http://en.wikiversity.org/wiki/Fourier_series#Plancherel.27s_and_Parseval.27s_theorems
At the bottom they have done that, but I'm not sure how they went from complex to real coefficients.
No idea where they got the $a^{2}_{0}$ from too.

3. The attempt at a solution
Any ideas would be appreciated.

Last edited: Oct 4, 2011