Proposition: 3.20: Inscribed Angle Theorem
(Proposition 20 from Book 3 of Euclid's “Elements”)
In a circle, the angle at the center is double that at the circumference, when the angles have the same circumference base.
Modern Formulation
The central angle is double the inscribed angle.
Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1 2 3
Sections: 4
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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