Proposition: 3.03: Conditions for Diameter to be a Perpendicular Bisector
(Proposition 3 from Book 3 of Euclid's “Elements”)
In a circle, if any straight line through the center cuts in half any straight line not through the center then it also cuts it at right angles. And (conversely) if it cuts it at right angles then it also cuts it in half.
Modern Formulation
Let $AB$ be a chord, which does not go through the center $E$ of a given circle and let $CD$ be its diameter, i.e. it goes through the center $E.$
Then $CD$ bisects $AB$ if and only if $CD\perp AB,$ i.e. both are perpendicular.
Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1 2 3 4 5
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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