Given: ad ? bc and ad ? bc prove: abcd is a parallelogram. statements reasons 1. ad ? bc; ad ? bc 1. given 2. ? cad and ? acb are alternate interior ? s 2. definition of alternate interior angles 3. ? cad ? ? acb 3. alternate interior angles are congruent 4. ac ? ac 4. reflexive property 5. ? cad ? ? acb 5. sas congruency theorem 6. ab ? cd 6. ? 7. abcd is a parallelogram 7. parallelogram side theorem what is the missing reason in step 6? sss congruency theorem cpctc definition of a parallelogram opposite sides in a parallelogram are congruent