Proposition: Prop. 11.05: Three Intersecting Lines Perpendicular to Another Line are in One Plane
(Proposition 5 from Book 11 of Euclid's “Elements”)
If a straight line is set up at right angles to three straight lines cutting one another, at the common point of section, then the three straight lines are in one plane.
* For let some straight line $AB$ have been set up at right angles to three straight lines $BC$, $BD$, and $BE$, at the (common) point of section $B$.
* I say that $BC$, $BD$, and $BE$ are in one plane.
Modern Formulation
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Table of Contents
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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