8.
如图,在五面体\(ABCDEF\)中,四边形\(ABCD\)是边长为\(4\)的正方形,\(EF{/\!/}AD\),平面\(ADEF\bot \)平面\(ABCD\),且\(BC=2EF\),\(AE=AF\),点\(G\)是\(EF\)的中点.
\((\)Ⅰ\()\)证明:\(AG\bot \)平面\(ABCD\);
\((\)Ⅱ\()\)若直线\(BF\)与平面\(ACE\)所成角的正弦值为\(\dfrac{\sqrt{6}}{9}\),求\(AG\)的长;
\((\)Ⅲ\()\)判断线段\(AC\)上是否存在一点\(M\),使\(MG/\!/\)平面\(ABF\)?若存在,求出\(\dfrac{AM}{MC}\)的值;若不存在,说明理由.