What theorem is dynamically being illustrated in the applet below? Describe in words. How can you formally prove it? Try to do so!
For more information, see this [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]).