Proposition: 1.16: The Exterior Angle Is Greater Than Either of the NonAdjacent Interior Angles
(Proposition 16 from Book 1 of Euclid's “Elements”)
For any triangle, when one of the sides is produced, the external angle is greater than each of the internal and opposite angles.
 Let $ABC$ be a triangle, and let one of its sides $BC$ have been produced to $D$.
 I say that the external angle $ACD$ is greater than each of the internal and opposite angles, $CBA$ and $BAC$.
Modern Formulation
Construct \(\triangle{ABC}\) and extend any of its sides, e.g. \(\overline{BC}\), to the segment \(\overline{CD}\). Then the exterior angle \(\gamma=\angle{DCA}\) is greater than either of the interior nonadjacent angles \(\alpha=\angle{CBA}\) and \(\beta=\angle{BAC}\).
Table of Contents
Proofs: 1 Corollaries: 1
Mentioned in:
Corollaries: 1
Proofs: 2 3 4 5 6 7 8 9
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References
Adapted from CC BYSA 3.0 Sources:
 Callahan, Daniel: "Euclid’s 'Elements' Redux" 2014
Adapted from (Public Domain)
 Casey, John: "The First Six Books of the Elements of Euclid"
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"