So, in the figure below, if , then
Since , by the Corresponding Angles Postulate,
Also, by the Vertical Angles Theorem,
Then, by the Transitive Property of Congruence,
You can prove that and are congruent using the same method.
The converse of this theorem is also true; that is, if two lines and are cut by a transversal so that the alternate exterior angles are congruent, then .