1. Problem 5.21 5.21 Apply Parseval's theorem to the result of problem 5.9 and see what you can deduce. 5.9 (a) Use the periodic boundary conditions on -L < x

< +L and basis e^ninx/L to write r² as a Fourier series. Sketch the sums up to a few terms. (b) Evaluate your result at x = L where you know the answer to be L² and deduce from this the value of ((2).