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如图,在梯形\(ABCD\)中,\(AB/\!/CD\),\(AD=DC=CB=1\),\(∠ABC=60^{\circ}\),四边形\(ACFE\)为矩形,平面\(ACFE⊥\)平面\(ABCD\),\(CF=1\).
\((\)Ⅰ\()\)求证:\(BC⊥\)平面\(ACFE\);
\((\)Ⅱ\()\)点\(M\)在线段\(EF\)上运动,设平面\(MAB\)与平面\(FCB\)所成二面角的平面角为\(θ(θ\leqslant 90^{\circ})\),试求\(\cos θ\)的取值范围.