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.
$m\angle 4=m\angle 1+m\angle 2$
Proof:
Given: $\Delta PQR$
To Prove: $m\angle 4=m\angle 1+m\angle 2$
Statement 
Reason 

1 
$\Delta PQR$ is a triangle

Given 
2 
$m\angle 1+m\angle 2+m\angle 3=180\xb0$ 
Triangle Sum Theorem 
3 
$\angle 3$ and $\angle 4$ form a linear pair 
Definition of linear pair. 
4 
$\angle 3$ and $\angle 4$ are supplementary 
If two angles form a linear pair, they are supplementary. 
5 
$m\angle 3+m\angle 4=180\xb0$ 
Definition of supplementary angles. 
6 
$m\angle 3+m\angle 4=m\angle 1+m\angle 2+m\angle 3$ 
Statements 2, 5 and Substitution Property. 
7 
$m\angle 4=m\angle 1+m\angle 2$ 
Subtraction Property. 