lax+1l≤b
-b≤ax+1≤b
-b-1≤ax≤b-1
(1)a＞0时
-(b+1)/a≤x≤(b-1)/a
-(b+1)/a=-1
(b-1)/a=3
b=-2,a=-1不满足假设 舍
(2)a=0时
-b<1不满足题意 舍
(3)a<0时
(b-1)/a≤x≤-(b+1)/a
-(b+1)/a=3
(b-1)/a=-1
b=2 a=-1