The exterior angle theorem is Proposition 1.16 of Euclid's Elements. It states that the measure of an exterior angle of a triangle is greater than the measure of either of the triangle's remote interior angles, the two interior angles not adjacent to that exterior angle. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/
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Source Exterior angle theorem (Wikipedia)
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1. Exterior angle theorem (Wikipedia)
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The exterior angle theorem is Proposition 1.16 in Euclid's Elements, which states that the measure of an exterior angle of a triangle is greater than either of the measures of the remote interior angles.
In Branch: Geometry, Lead paragraph
This is a fundamental result in absolute geometry because its proof does not depend upon the parallel postulate.
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