external angle vs exterior angle

external angle and exterior angle both are nouns.

Word	Noun	Adjective	Verb	Adverb
external angle	Yes	No	No	No
exterior angle	Yes	No	No	No

As nouns, external angle and exterior angle are synonyms defined as:

external angle and exterior angle: the supplement of an interior angle of a polygon

external angle (noun)	exterior angle (noun)
the supplement of an interior angle of a polygon	the supplement of an interior angle of a polygon