9.
如图所示,在直四棱柱\(ABCD\)\(-\)\(A\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\) \(C\)\({\,\!}_{1}\)\(D\)\({\,\!}_{1}\)中,已知\(AD=1\),且\(DC\)\(=\)\(DD\)\({\,\!}_{1}=2\)\(AD\)\(=2\)\(AB\),\(AD\)\(⊥\)\(DC\),\(AB\)\(/\!/\)\(DC\).
\((1)\)设\(E\)是\(DC\)的中点,求证:\(D\)\({\,\!}_{1}\)\(E\)\(/\!/\)平面\(A\)\({\,\!}_{1}\)\(BD\);
\((2)\)求二面角\(A\)\({\,\!}_{1}—\)\(BD\)\(—\)\(C\)\({\,\!}_{1}\)的余弦值.
\((3)\) 求直线与平面\(C\)\({\,\!}_{1}\)\(BD\)所成角的正弦值.