如图,四棱柱ABCD-A1B1C1D1的所有棱长都相等,AC∩BD=O,A1C1∩B1D1=O1,四边形ACC1A1和四边形BDD1B1为矩形. (1)证明:O1O⊥平面;ABCD
(2)若∠CBA=60°,求二面角C1-OB1-D的余弦值.
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