Theorem 9

If two parallel lines are cut (crossed) by a transversal, then their corresponding angles are congruent

Theorem 10

If the two angles in a linear pair have equal measures (are congruent), then each is a right angle

Theorem 11

If two parallel lines are cut (crossed) by a transversal, then their alternate exterior angles are congruent

Theorem 12

If two parallel lines are cut (crossed) by a transversal, then interior angles on the same side of the transversal are supplementary

Theorem 13

If two lines form congruent corresponding angles with a transversal, then the lines are parallel

Theorem 14

If two lines form congruent alternate exterior angles with a transversal, then the lines are parallel

Theorem 15

If two lines form supplementary interior angles on the same side of the transversal, then the lines are parallel