Proposition: 3.18: Radius at Right Angle to Tangent
(Proposition 18 from Book 3 of Euclid's “Elements”)
If some straight line touches a circle, and some (other) straight line is joined from the center (of the circle) to the point of contact, then the (straight line) so joined will be perpendicular to the tangent.
Modern Formulation
If a tangent and a radius of a given circle intersect, then they are perpendicular.
Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1 2 3 4 5 6
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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