如图,在矩形ABCD中,点E,F分别在线段AB,AD上,AE=EB=AF=
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FD=4.沿直线EF将△AEF翻折成△A′EF,使平面A′EF⊥平面BEF.
(Ⅰ)求二面角A′-FD-C的余弦值;
(Ⅱ)点M,N分别在线段FD,BC上,若沿直线MN将四边形MNCD向上翻折,使C与A′重合,求线段FM的长.
考点分析:
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