选择题：直三棱柱 ABC﹣ A 1 B 1 C 1 中， AA 1 B 1 B 为正方形， AB＝BC， ∠ABC＝120° ， M为棱 BB 1 上任意一点， 点 D

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直三棱柱 ABC﹣ A 1 B 1 C 1 中， AA 1 B 1 B 为正方形， AB＝BC， ∠ABC＝120° ， M为棱 BB 1 上任意一点， 点 D、 E 分别为 AC、 CM的中点．

（1） 求证： DE∥平面 AA 1 B 1 B；

（2） 当点 M为 BB 1 中点时， 求直线 B 1 C 和平面 CDM所成角的正弦值．

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