12.已知正三棱柱ABC−A1B1C1ABC-A_{1}B_{1}C_{1}ABC−A1B1C1的所有棱长均相等,点MMM为A1B1A_{1}B_{1}A1B1的中点,则异面直线AMAMAM与B1CB_{1}CB1C所成角的余弦值为((( )))