Showing that three points are collinear if their cross product is zero

[hachi_roku]

Homework Statement

show that three points P,Q, and R are collinear if and only if vec PQ x vec PR = 0

Homework Equations

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?

 

[rock.freak667]

axb=|a||b|sinθ

so what is the angle between PQ and PR?

 

[hachi_roku]

thanks haha =)