Explanation:
The sides of a rhombus are all the same length, by definition, so ΔACB ≅ ΔACD by SSS, so ∠ACB ≅ ∠ACD by CPCTC. Those equal angles sum to ∠BAD by the angle sum theorem, so AC is a bisector of ∠BAD by the definition of an angle bisector.
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The measure of ∠BAD is irrelevant..
