Proposition: Prop. 11.20: Sum of Two Angles of Three containing Solid Angle is Greater than Other Angle
(Proposition 20 from Book 11 of Euclid's “Elements”)
If a solid angle is contained by three rectilinear angles then (the sum of) any two (angles) is greater than the remaining (one), (the angles) being taken up in any (possible way).
 For let the solid angle $A$ have been contained by the three rectilinear angles $BAC$, $CAD$, and $DAB$.
 I say that (the sum of) any two of the angles $BAC$, $CAD$, and $DAB$ is greater than the remaining (one), (the angles) being taken up in any (possible way).
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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