By the definition of a , AD / BC and AB / DC.
Using, AD as a transversal, ZA and are same-
side interior angles, so they are
Using side as a transversal, ZB and Care
same-side interior angles, so they are supplementary.
Using AB as a transversal, ZA and B are same-side
interior angles, so they are supplementary.
Therefore, ZA is congruent to ZC because they are
supplements of the same angle. Similarly, ZB is
congruent to Z