Creation of this resource was inspired by a [url=http://www.gogeometry.com/problem/p284_circular_sector_semicircle_tangent_radius.htm]problem[/url] posted by [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry).[br][br]You can move the LARGE POINT (bottom right) if you'd like.[br][b][br]How can we formally prove what is dynamically illustrated here?[/b]