第 015 题 / 90

如图,三棱锥 $A - {BCD}$ 中,
${AB} = {AC} = {BD} = {CD} = 3,{AD} = {BC} = 2$ ，点 $M,N$ 分别是 ${AD},{BC}$ 的中点，则异面直线 ${AN},{CM}$ 所成的角的余弦值是___.