如图,在四棱柱\(ABCD-A_{1}B_{1}C_{1}D_{1}\)中,侧面\(ADD_{1}A_{1}⊥\)底面\(ABCD\),\(D_{1}A=D_{1}D= \sqrt {2}\),底面\(ABCD\)为直角梯形,其中\(BC/\!/AD\),\(AB⊥AD\),\(AD=2AB=2BC=2\),\(O\)为\(AD\)中点.
\((\)Ⅰ\()\)求证:\(A_{1}O/\!/\)平面\(AB_{1}C\);
\((\)Ⅱ\()\)求锐二面角\(A-C_{1}D_{1}-C\)的余弦值.