# G.1.21.3 Triangle Angle Sum Another Way

- Author:
- Katie Akesson

Triangle *ABC* has been translated along the directed line segment from *B* to *C* to make triangle *A'B'C'* and triangle *ABC *was also translated along the directed line segment from *A* to *C* to make triangle *A''B''C''*.
Use what you know about vertical angles to prove the Triangle Angle Sum Theorem.