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).

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