Given: ABCD is a parallelogram. Prove: ∠A ≅ ∠C and ∠B ≅ ∠D
Parallelogram A B C D is shown.
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 ∠.