5.
如图,正三棱柱\(ABC\)\(-\)\(A\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\)\(C\)\({\,\!}_{1}\)的所有棱长都为\(2\),\(D\)为\(CC\)\({\,\!}_{1}\)中点.
\((1)\)求证:\(AB\)\({\,\!}_{1}⊥\)平面\(A\)\({\,\!}_{1}\)\(BD\);
\((2)\)求二面角\(A\)\(-\)\(A\)\({\,\!}_{1}\)\(D\)\(-\)\(B\)的余弦值;
\((3)\)求点\(C\)\({\,\!}_{1}\)到平面\(A\)\({\,\!}_{1}\)\(BD\)的距离.