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题 如图,在正三棱柱ABC-A<sub>1</sub>B<sub>1</sub>C<sub>1</sub>中,E∈BB<sub>1</sub>,截面A<sub>1</sub>EC⊥上侧面 AC<sub>1</sub>. (Ι)求证:BE=EB<sub>1</sub>; (Ⅱ)若AA<sub>1</sub>