Creation of this applet was inspired by a [url=https://twitter.com/solvemymaths/status/1001165579879841794]problem[/url] posted by[url=https://twitter.com/solvemymaths] Ed Southall[/url]. [br][br][b]LARGE POINTS are moveable. [br][color=#cc0000]RED POINT = center of red circle.[/color][br][color=#9900ff]Purple point = center of purple circle. [/color] [/b][br][br]How can we formally prove what is dynamically illustrated here? [br]