GoGeometry Action 135!

Creation of this applet was posed by a problem posted by Antonio Gutierrez (GoGeometry). Note the quadrilateral below is split into 2 triangles. The incircles of both triangles are shown. The blue points are points of tangency. Slide the slider carefully and observe what happens. Then, answer question (1) below. How can we formally prove what is dynamically illustrated here?


Suppose the 4 sides of this quadrilateral have lengths p, q, r, and s. Express the distance between the 2 blue points in terms of p, q, r, and s.

Quick (Silent) Demo