The measure of an exterior angle of a triangle is equal to the sum of the measures of the two nonadjacent 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. 