连接AB1,AC1 由题意知,点M,N分别为AB1和B1C1的中点,∴MN∥AC1 又MN平面A1ACC1,AC1平面A1ACC1 ∴MN∥平面A1ACC1 取A1B1中点P,连MP,NP,而M,N 分别为AB1与B1C1的中点,∴MP∥AA1,MP平面A1ACC1,AA1平面A1ACC1∴MP∥平面A1ACC1,同理可证NP∥平面A1ACC1 又MP∩NP=P∴平面MNP∥平面A1ACC1 ∵MN平面MNP ∴MN∥平面A1ACC1