Please install Java (version 1.4 or later) to use JavaSketchpad applets.

Cyclic Quadrilateral Arcs Theorem

For a further hint go to A Further Hint

If the above hints don't help: A proof of this result can be found on p. 190 of my Some Adventures in Euclidean Geometry book which is available for purchase as downloadable PDF or printed book at Further Info

NOTE: Due to the way the Sketchpad sketch was constructed & how some constructions are defined in the program, the above sketch might suddenly show the two lines parallel when the cyclic quadrilateral is dragged until it becomes "crossed". Have a look at the Cinderella sketch below that doesn't show this "discontinuity".(Drag A, B, C or D).

Please enable Java for an interactive construction (with Cinderella).