Exterior Angle Theorem

The measure of an exterior angle of a triangle is equal to the sum of the measures of the two non-adjacent interior angles of the triangle.

Proof:

Given:PQR

To Prove:

Statement
Reason
1

PQR is a triangle

Given
2
Triangle Sum Theorem
3
form a linear pair
Definition of linear pair.
4
are supplementary
If two angles form a linear pair, they are supplementary.
5
Definition of supplementary angles.
6
Statements 2, 5 and Substitution Property.
7
Subtraction Property.