# Ptolemy Theorem - Proof Without Words

In a cyclic ABCD quadrilateral with sides a, b, c, d, and diagonals e and f, the product of diagonals equals the sum of the products of the opposite sides:

### References

- W. Derrick, J. Herstein,
__Proof Without Words: Ptolemy's Theorem__,*The College Mathematics Journal*, v 43, n 5, November 2012, p 386

### Ptolemy's Theorem

- Ptolemy's Theorem
- Sine, Cosine, and Ptolemy's Theorem
- Useful Identities Among Complex Numbers
- Ptolemy on Hinges
- Thébault's Problem III
- Van Schooten's and Pompeiu's Theorems
- Ptolemy by Inversion
- Brahmagupta-Mahavira Identities
- Casey's Theorem
- Three Points Casey's Theorem
- Ptolemy via Cross-Ratio
- Ptolemy Theorem - Proof Without Word
- Carnot's Theorem from Ptolemy's Theorem

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