Creation of this applet was inspired by a [url=http://www.gogeometry.com/problem/p763-parallelogram-diagonal-parallel-congruence-plane-geometry.htm]post[/url] from Antonio Gutierrez ([url=http://www.gogeometry.com/]GoGeometry[/url]). [br][br][b][color=#1e84cc]Within this context, how can we formally prove the 2 blue segments congruent? [/color][/b] [br]