1. The problem statement, all variables and given/known data Studying for an exam, this is a question from an earlier exam: Calculate: sum k from 1 to infinity, 1/k^2 I have from previous question: f(t)=pi+sum n from 1 to infinity: 2*(-1)^n/n*sin(nt) f(t)=pi-t, -pi<=t<pi So i need to rearrange this to calc the sum. 2. Relevant equations 3. The attempt at a solution calc f(t) in a good point, I choose pi/2, use the left and right limit to get pi/2 then try top rearrange the series. Especially I want to know mhow to do Sin(n*pi/2). it makes every second term disappear in the series but I dont know how to do this right. I tried replacing with n=2k+1 but it doesnt work.