13. 正方体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,MMM、NNN分别为A1B1A_{1}B_{1}A1B1、A1D1A_{1}D_{1}A1D1的中点,EEE、FFF分别是B1C1B_{1}C_{1}B1C1、C1D1C_{1}D_{1}C1D1的中点.(1)求证:EEE、FFF、BBB、DDD共面;(2)求证:平面AMN//AMN//AMN//平面EFDBEFDBEFDB.