Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/810859898569424896]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url]. [br][br][b][color=#1e84cc]Feel free to move points [i]A[/i] and [i]B[/i] wherever you'd like[/color][/b] [b]prior to sliding the slider. [/b] [br][b]Be sure to change the location of point [i]E[/i] (soon to appear) as well. [br][/b][br][i]How can you formally prove what this applet informally illustrates?[/i]