**a) Prove that if and 4|n, then x, y, and z are even.** **b) Prove that if n is of the form , then n is NOT the sum of three squares.**
===================================

How can we prove part a?

For part b, this is what I've got so far.

Suppose n is a sum of three squares

(aim for a contradiction). Assuming the result of part a, since 4|

, x,y, and z must be even, so x/2, y/2, z/2 are integers.

=>

, so n/4 is also a sum of three squares

How to finish the proof from here?

Any help is appreciated!

[note: also under discussion in math links forum]