Ptolomy's theorem

Very similar in context to both intersecting chords and cyclic quadrilateral, Ptolomy's theorem basically states that the sum of the products of opposite sides is eqaul to the product of the diagonals.

The proof is a bit harder and so is included as a second file below (and makes much use of angles in the same segment).

The converse of this theorem is also true.

To download this Geogebra file click here.

To download this Geogebra file click here.