Creation of this applet was posed by a [url=http://www.gogeometry.com/school-college/4/p1308-quadrilateral-diagonal-incircle-tangent.htm]problem[/url] posted by [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br]Note the quadrilateral below is split into 2 triangles. The incircles of both triangles are shown. [br]The [b][color=#1e84cc]blue points [/color][/b]are points of tangency. [br][br]Slide the slider carefully and observe what happens. [br]Then, answer question (1) below. [br][br][b]How can we formally prove what is dynamically illustrated here?[/b]