Let $V$ be the real inner product space. Prove that for every $x, y \in V$, it holds $||x+y||^2 + ||x-y||^2 = 2(||x||^2 + ||y||^2)$
Maybe I don't have a lot of knowledge in inner product space . I have read the definition and work at some problems in my textbook, but I can't solve this one unfortunately. Could you help me?