⑴ BB1⊥A1B1C1D1 EF∈A1B1C1D1 ∴BB1⊥EF EF⊥BB1
EF∥A1C1﹙中位线﹚ A1C1⊥B1D1 ∴EF⊥B1D1
∵ EF⊥BB1 EF⊥B1D1 ∴EF⊥BB1D1D
⑵ ABCD∥A1B1C1D1 OP与DM相交,确定一个平面α DP∈α∩ABCD OM∈α∩A1B1C1D1
∴DP∥OM.
⑴ BB1⊥A1B1C1D1 EF∈A1B1C1D1 ∴BB1⊥EF EF⊥BB1
EF∥A1C1﹙中位线﹚ A1C1⊥B1D1 ∴EF⊥B1D1
∵ EF⊥BB1 EF⊥B1D1 ∴EF⊥BB1D1D
⑵ ABCD∥A1B1C1D1 OP与DM相交,确定一个平面α DP∈α∩ABCD OM∈α∩A1B1C1D1
∴DP∥OM.