证明：（I）∵A1D1⊥平面A1B1BA，EF⊂平面A1B1BA，∴A1D1⊥EF ∵A1B⊥AB1，EF∥AB1，∴A1B⊥EF∴EF⊥平面A1D1B；（II）延长DE、CB交于N，∵E为AB中点，∴△DAE≌△NBE过B作BM⊥EN交于M，连FM，∵FB⊥平面ABCD∴FM⊥D...