Creation of this applet was inspired by a [url=http://www.gogeometry.com/problem/p219_rhombus_perpendicular_arithmetic_mean.htm]post[/url] from Antonio Gutierrez ([url=http://www.gogeometry.com/]GoGeometry[/url]). [br][br]Once the [b][color=#666666]GRAY POINT[/color][/b] appears (on the left side), [b][color=#666666]feel free to move it along that side.[/color][/b][br][b][color=#38761d]You can control the size of the green angle by using the green slider. [/color][/b][br][color=#0000ff][b][br]How can we formally prove what this applet informally illustrates? [/b][/color]