It is true and proved that p║q using the corresponding angle theorem.
What are supplementary angles.
Supplementary angles are angles that complement themselves to make 180 degrees, it means that the addition of these two angles results into 180 degrees.
If there are two parallel line p and q, cut by a transversal line r. Then the alternate exterior angle ∠1 and interior angle ∠3 are equal using the corresponding angle theorem.
Also, ∠1 and ∠2 are supplementary angles if the sum of angle ∠3 and ∠2 = 180 degrees. Thus, the sum ∠1 and ∠4(opposite exterior angle on line p) is equal to the sum of ∠3 and ∠2 on line q.
Then it is proved that p║q.