1. The problem statement, all variables and given/known data show that three points P,Q, and R are collinear if and only if vec PQ x vec PR = 0 2. Relevant equations 3. The attempt at a solution i understand this (i can see this visually in my head) but i just don't know how to say it mathematically...i know that the angle between them must be 0 or 180, but how do i show this????