∵AA'//DD' ∴ AA’//平面B’D'D
作AE⊥BD于E
∵AA'//DD' ∴ AA’//平面B’D'D
∵平面ABCD⊥平面B’D'D ∴ AE⊥平面B’D'D
在Rt△ABD,求出BD=17,AE=15*8/17=120/17即可.