Euclid's Elements & Constructions – Lecture 9 (Geometry)

An exterior angle of a triangle is defined as an angle that is:

1. A.Vertical to one of the triangle's interior angles.
2. B.Adjacent and supplementary to an angle of the triangle.
3. C.Formed by extending both sides of one vertex outward.
4. D.Equal to the sum of all three interior angles.