External Angle of Triangle equals Sum of other Internal Angles

Theorem
The external angle of a triangle equals the sum of the other two internal angles.

Also known as
This theorem is referred to by some sources as the Interior Angles Theorem, and by others as the Exterior Angle Theorem. These names, however, are also used to refer to other theorems.

In the wake of such possible confusion, it is prudent not to refer to it by name at all, but by description, as has been done on.