Final answer:
The Alternate Exterior Angles Theorem can be used to prove that alternate exterior angles are congruent when two parallel lines are intersected by a transversal.
Step-by-step explanation:
Alternate exterior angles are congruent due to the Alternate Exterior Angles Theorem. According to this theorem, when two parallel lines are intersected by a transversal, the alternate exterior angles formed are congruent.
For example, consider two parallel lines, line a and line b, intersected by a transversal line c. Alternate exterior angles are formed on opposite sides of the transversal and on the exterior of the parallel lines. These angles will always have the same measure.
By applying the Alternate Exterior Angles Theorem, it can be proven that alternate exterior angles are congruent in any given scenario.