8."a=−3a=-3a=−3"是直线l1:ax+(1−a)y=3l_{1}:ax+(1-a)y=3l1:ax+(1−a)y=3与l2:(a−1)x+(2a+3)y=2l_{2}:(a-1)x+(2a+3)y=2l2:(a−1)x+(2a+3)y=2互相垂直的((( )))