Suppose that u is a non-zero vector in R^n.

Prove that ||u-v|| = ||u|| - ||v||

if and only if v = ku for k is ubset of R with 0 <= k <= 1.

v and u are vectors. k is a scalar

I subbed v=ku into the right side, and I got

||u|| - k ||u|| as my last step for the right side, but I have no idea how to make it equal to the left side