# Ptolemy Theorem

- Author:
- Terry Tam

- Topic:
- Geometry

ABCD is a cyclic quadrilateral.
a) A and A' are the inversion points of each other. Prove that and
b) Applying result of a) on B and C (their corresponding inversion points are B' and C' with the same inversion circle), prove that and express B'C' in terms of r and sides of .
c) Applying the above results to another two triangles, prove the Ptolemy Theorem.

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