Creation of this applet was inspired by a [url=http://www.gogeometry.com/problem/p768-parallelograms-equilateral-triangles-high-school-college.htm]post[/url] from Antonio Gutierrez ([url=http://www.gogeometry.com/]GoGeometry[/url]). [br][br]Feel free to move the original white points anywhere you'd like.[br][color=#ff00ff]You can also change the size of the pink angle by using the pink slider. [br][/color][br][color=#0000ff][b]How can we formally prove what this applet informally illustrates? [/b][/color]