Please help me with the following questions.

Problem 2. Let V be a finite-dimensional inner product space and S = {ul, ..., Un} be an orthonormal set of vectors in V. Prove the following: 1. (| |2 > > K(x, ui)| for all x EV (this is called Bessel's inequality). 2. Wall? = ELK(x, ui)|" if and only if a E span(S) (this is a generalized version of the Pythagorean Theorem, sometimes called Parseval's identity)