By the definition of a ▱, ad∥bc and ab∥dc. using, ad as a transversal, ∠a and ∠ are same-side interior angles, so they are . using side as a transversal, ∠b and ∠c are same-side interior angles, so they are supplementary. using ab as a transversal, ∠a and ∠b are same-side interior angles, so they are supplementary. therefore, ∠a is congruent to ∠c because they are supplements of the same angle. similarly, ∠b is congruent to ∠.