# 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

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