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.
![fig20e](https://github.com/bookofproofs/bookofproofs.github.io/blob/main/_sources/_assets/images/euclid/Book03/fig20e.png?raw=true)
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 BY-SA 3.0 Sources:
- Prime.mover and others: "Pr∞fWiki", https://proofwiki.org/wiki/Main_Page, 2016