9.
如图,边长为\(\sqrt{2}\)的正方形\(ADEF\)与梯形\(ABCD\)所在的平面互相垂直,其中\(AB/\!/CD\),\(AB\bot BC\),\(CD=BC=\dfrac{1}{2}AB=1\),\(AE\bigcap DF=O\),\(M\)为\(EC\)的中点.
\((\)Ⅰ\()\)证明:\(OM/\!/\)平面\(ABCD\);
\((\)Ⅱ\()\)求二面角\(D-AB-E\)的正切值;
\((\)Ⅲ\()\)求\(BF\)与平面\(ADEF\)所成角的余弦值.