# find a point on a line?

Hi! I have 5 points, stored in a vector<point2f>, that i know the coordinates of. The 5th point is on a line between 2 of the others. What is the best way to find out which two? And which one of the two it is closest to?

Many thanks.

c++

Do all of the 5 points fall on the same line?

no, four of them are corners of a square, and the other is somewhere along the edge of the square.

thanks!