14.如图,设正方体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1的棱长为aaa.则顶点B1B_{1}B1到面BA1C1BA_{1}C_{1}BA1C1的距离为((( )))