This is not really a homework problem, but I'm studying a text, and I came across this: http://img198.imageshack.us/img198/4586/sumh.jpg [Broken] I know how to get that fraction with the exponents in it (using a summation formula). But for the life of me, I can't figure out how to manipulate that fraction to give the final result. For example, if I put k=0 into that fraction, I get 0/0, not 5. I tried a bunch of manipulation of the fraction to get sines and cosines, and make the denominator real, but I still can't get a closed form solution that gives the final result. What am I missing? Thanks!! UPDATE: Forgot to mention, this is a discrete time function. k is always an integer.