Corollary to Theorem 2.3: (The midpoint formula) Let (x1, y1) and (x2, y2) be two points. The midpoint between them has coordinates
Proof: We will define the midpoint as the point on the line between (x1, y1) and (x2, y2) whose distance from each point is half of the total distance between them. By Theorems 2.1 and 2.2 such a point is obtained by letting t = 1/2 in Theorem 2.1. In that case, 1 - t = 1/2, and the result follows.
next theorem (2.4)