由A:x-3>0,x+a<0,得3<x<-a,(a<-3时要)x-3<0,x+a>0,得-a<x<3(a>-3时)由B:x²-3x-4<0(x+1)(x-4)<0,-1<x<4,取x=0,1,2,3.求交集?并集?