exterior angles of triangle
Proof. Let us study in an arbitrary triangle for example the exterior angle where is point on the lengthening of the side nearer to than to . Let be the midpoint of . Let be the median of the triangle. We find on its lengthening the point such that . Then the triangles and are congruent (SAS). Consequently, we have and therefore . Analogically one shows that .
- 1 Karl Ariva: Lobatsevski geomeetria. Kirjastus “Valgus”, Tallinn (1992).