Interior and Exterior Angles of Triangles
You have learned that the measures of the interior angles in any triangle sum up to This theorem is called Triangle Angle-Sum Theorem.
Theorem. Triangle Angle-Sum Theorem
The sum of the measures of the interior angles in any triangle is
The Exterior Angle Theorem is useful in proving the Triangle Angle-Sum Theorem.
| Given: | with interior angles 1, 2, and 3 |
| Prove: |
Proof:
