Step-by-step explanation:
The proof can make use of the ASA congruence postulate.
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Statement . . . . Reason
AB║CD, AD║BC . . . . definition of parallelogram
∠ACD ≅ ∠CAB, ∠ACB ≅ ∠CAD . . . . alternate interior angles theorem
ΔACD ≅ ΔCAB . . . . ASA congruence postulate