8.如图,在直三棱柱ABC−A1B1C1ABC-A_{1}B_{1}C_{1}ABC−A1B1C1中,AC⊥BCAC\bot BCAC⊥BC,AC=CC1=2BCAC=CC_{1}=2BCAC=CC1=2BC,则直线BC1BC_{1}BC1与直线AB1AB_{1}AB1夹角的余弦值为((( )))