如图取坐标系D﹙0,0,0﹚ A﹙2,0,0﹚ C﹙0,2,0﹚.D1﹙0,0,2﹚
则DB1=﹛1.1.2﹜
BC1=﹛-2.-1,2﹜
cos[DB1与BC1夹角]=DB1•BC1/[|DB1|×|BC1|]=1/﹙3√6﹚