Geometry · Triangles & Congruence
Exterior Angle Theorem
HSG-CO.C.10
Definition
An exterior angle of a triangle equals the sum of the two non-adjacent (remote) interior angles.
Examples
- 50 + 60 = 110 degrees, so the exterior angle measures 110 degrees.
- If the remote interior angles are 40 and 75 degrees, the exterior angle is 115 degrees.
Key rule
An exterior angle always equals the sum of the two remote interior angles.
Memory hook
The outside angle equals the two far-away inside angles added up.