This problem comes from a second-semester course in linear algebra. We are currently covering Gram Scmidt - but outside of the application problems, I'm a bit lost.

Problem: Suppose V is an inner product space and {v_1, ... , v_k} is an orthonormal set of vectors in V. Show that for all v in V, we have:

.

Furthermore, show that the equality holds if and only if v is in the span of {v_1, ... , v_k}.

Intuition: I can't picture the problem in anything larger than 2-space. If in two space, it seems plausible that a right triangle could be formed using the orthonormal set and v_j (which would be the hypotenuse). I'm not sure if this stems from a theorem, or if I should just be playing with the definitions in the summation/inequality.