Creation of this resource was inspired by this [url=http://www.gogeometry.com/school-college/3/p1280-quadrilateral-perpendicular-diagonal-collinear-tutoring.htm]problem[/url] posted by [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br]You can move any of the [b]LARGE POINTS[/b] anywhere you'd like[b] AT ANY TIME[/b]. [br]As you do, notice that the diagonals of this quadrilateral always remain perpendicular. [br][br][b][color=#ff7700]How can we formally prove the 3 orange points are ALWAYS COLLINEAR? [/color][/b]