Answer:
If two lines are cut by a transversal such that alternate exterior angles are congruent, then the lines are parallel.
Step-by-step explanation:
Start by stating that the given angles are congruent.
Call the angle vertical to angle 1 "angle 3."
Then angle 3 and angle 1 are congruent by vertical angles.
Angle 3 and angle 2 are congruent by transitive congruence of angles.
That makes lines u and v parallel by congruent corresponding angles of two lines and a transversal.
Answer
You can prove the following theorem:
If two lines are cut by a transversal such that alternate exterior angles are congruent, then the lines are parallel.
