可以把A1B向下平移至AB2点
,AB2=A1B=根号5AB
AD1=根号5AB
D1B2^2=D1B1^2+B1B2^2=2AB^2+16AB^2=18AB^2
cos角D1AB2=(AB2^2+AD1^2-D1B2^2)/2AB2*AD1
=-8AB^2/10AB^2
=-4/5
则异面直线A1B与AD1所成角的余弦值为-4/5