Explanation:
Statements are numbered; reasons are in italics.
1. ABCD is a parallelogram with AB≅CD and BC≅AD. Given; definition of a parallelogram.
2. Diagonal AC ≅ diagonal CA. Reflexive property of congruence.
3. ΔABC ≅ ΔCDA. SSS congruence postulate.
4. ∠B ≅ ∠D. CPCTC. (Opposite angles B and D are congruent.)
5. Diagonal BD ≅ diagonal DB. Reflexive property of congruence.
6. ΔABD ≅ ΔCDB. SSS congruence postulate.
7. ∠A ≅ ∠C. CPCTC. (Opposite angles A and C are congruent.)