# Euclid, Book 3, Proposition 22

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Euclid's Proposition 22 from Book 3 of *The Elements* states that in a cyclic quadrilateral, opposite angles sum to 180°.

Contributed by: Jon Perry (March 2011)

Open content licensed under CC BY-NC-SA

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"Euclid, Book 3, Proposition 22"

http://demonstrations.wolfram.com/EuclidBook3Proposition22/

Wolfram Demonstrations Project

Published: March 7 2011