如图,在四棱锥 P-ABCD 中,已知 PB ⊥底面 ABCD , BC ⊥ AB , AD ∥ BC , AB = AD =2, CD ⊥ PD ,异面直线 PA 和 CD 所等于60°. (1)求证:面 PCD ⊥面 PBD ; (2)求直线 PC 和平面 PAD 所的正弦值的大小; (3)在棱 PA 上是否存在一点 E ,使得二面角 A-BE-D 的余弦值为 ?若存在,指出点 E 在棱 PA 上的位置,若不存在,说明理由.
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