1. 在长方体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,已知AA1=2AD=2CDAA_{1}=2AD=2CDAA1=2AD=2CD,点EEE是线段CDCDCD的中点,则异面直线D1ED_{1}ED1E与BC1BC_{1}BC1所成角的余弦值为((( )))