6. 正方体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,MMM,NNN分别是A1D1A_{1}D_{1}A1D1,B1C1B_{1}C_{1}B1C1的中点,则异面直线AMAMAM与CNCNCN所成角的余弦值为((( )))