help me长方体ABCD-A1B1C1D1中,E是BC的中点,M,N分别是AE,CD1的中点,AD=AA1=a,AB=2a （1）求证：MN‖平面ADD1A1（2）求异面直线AE和CD1所成角的余弦值 （长方体是横着放的,下面的底面是ABCD,上面的长方形是A

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