如图,在三棱柱\(ABC-A_{1}B_{1}C_{1}\)中,侧棱\(CC_{1}⊥\)地面\(ABC\),且\(CC_{1}=2AC=2BC\),\(AC⊥BC\),\(D\)是\(AB\)的中点,点\(M\)在侧棱\(CC_{1}\)上运动.
\((1)\)当\(M\)是棱\(CC_{1}\)的中点时,求证:\(CD/\!/\)平面\(MAB_{1}\);
\((2)\)当直线\(AM\)与平面\(ABC\)所成的角的正切值为\( \dfrac {3}{2}\)时,求二面角\(A-MB_{1}-C_{1}\)的余弦值.