1. The problem statement, all variables and given/known data Prove that |x+y|2 + |x-y|2 = 2|x|2 + 2|y|2 if x and y are elements of ℝk 2. Relevant equations Ordinary stuff 3. The attempt at a solution Got the first part, but I don't understand the parallelogram thing yet. The little sketch I drew didn't help.