若-a-1<-3a+1
则a+1>3a-1
a>1
若-a-1=-3a+1
2a=2
a=1
若-a-1>-3a+1
则a+1<3a-1
a<1
得到综合的结果,当a<1时,-a-1>-3a+1,当a=1时,-a-1=-3a+1,当a>1时,-a-1<-3a+1