N∈AC,则平面BNC1∩平面ACC1A1=C1N,平面AB1M1∩平面ACC1A1=AM,∵平面AB1M ‖ 平面BC1N ∴AM//NC1,（若两平面平行,且与第三平面相交,则其二交线也平行）,∵A1C1//AC,∴四边形ANC1M是平行四边形.∴MC1=AN,∵MC1=A1C1/2...