Creation of this applet was inspired by a [url=http://www.gogeometry.com/problem/p779-triangle-altitude-orthocenter-midpoint-90-degrees-high-school-college.htm]post[/url] from Antonio Gutierrez ([url=http://www.gogeometry.com/]GoGeometry[/url]). [br][br][b]You can move the vertices of the triangle anywhere you'd like. [/b][br][color=#1e84cc][b]You can also control the size of the blue angle by using the blue slider. [/b][/color][br][br][color=#9900ff][b]How can we formally prove what this applet informally illustrates? [/b][/color]