Alternate Exterior Angles Theorem
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 .