6.
\(18.\)如图,三棱柱
\(ABC\)\(\)
\(A\)\({\,\!}_{1}\)
\(B\)\({\,\!}_{1}\)
\(C\)\({\,\!}_{1}\)中,侧面
\(BB\)\({\,\!}_{1}\)
\(C\)\({\,\!}_{1}\)
\(C\)为菱形,
\(AB\)\(⊥\)
\(B\)\({\,\!}_{1}\)
\(C\).
\((1)\)证明:\(AC\)\(=\)\(AB\)\({\,\!}_{1}\);
\((2)\)若\(AC\)\(⊥\)\(AB\)\({\,\!}_{1}\),\(∠\)\(CBB\)\({\,\!}_{1}=60^{\circ}\),\(AB\)\(=\)\(BC\),求二面角\(A\)\(\)\(A\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\)\(C\)\({\,\!}_{1}\)的余弦值.