Proposition: 3.22: Opposite Angles of Cyclic Quadrilateral
(Proposition 22 from Book 3 of Euclid's “Elements”)
For quadrilaterals within circles, the (sum of the) opposite angles is equal to two right angles.
Modern Formulation
In a quadrilateral built upon four secants of a given circle, the sum of the opposite angles is $180^\circ$.
Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1 2
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References
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"
Adapted from CC BYSA 3.0 Sources:
 Prime.mover and others: "Pr∞fWiki", https://proofwiki.org/wiki/Main_Page, 2016