If two lines are intersected by a transversal, then alternate
interior angles, alternate exterior angles, and corresponding
angles are congruent. The converse of the theorem is true as
well. If two corresponding angles are congruent, then the two
lines cut by the transversal must be parallel. Similarly, if two
alternate interior or alternate exterior angles are congruent,
the lines are parallel. So the question is, if we have two lines
that might be parallel and they're intersected by a transversal,
can we do the converse of the parallel lines theorem? Which
says, if we have alternate interior angles or alternate exterior
angles, or corresponding angles that are congruent, is that
enough to say that these two lines are parallel? And as we read
right here, yes it is. If two lines and a transversal form
alternate interior angles, notice I abbreviated it, so if these
alternate interior angles are congruent, that is enough to say
that these two lines must be parallel. If you have alternate
exterior angles. That i