Checking if a Point is on a Line - Jay's Query

[inadaze]

Hey,
I was wondering:
If I had two points connected by a line on a cartesian plane(x and y for this) and i wanted to draw a third point, how could i tell if the third point was drawn somewhere on the line? I am programing a game where there is already a line drawn to connect to points and the user is supposed to draw a third point. i want to be able to check if it was drawn on the line or not.

Thanks
Jay

 

[nolachrymose]

Let p1 and p2 be the original two points, and p3 the third point. If the slopes between p1 and p2 is equal to that between p1 and p3 (or p2 and p3), then the three points lie on the same line.