A:6和8
B:11和9
C:12和17
D:13和8
I like this pair of boots, can I try ____on? (it)
(Ⅰ)求证:AC⊥平面ABB1A1;
(Ⅱ)求二面角A﹣C1D﹣C的平面角的余弦值.