x^2-2X-3<=0 =>-1<=x<=3|x-a|<=2 =>a-2<=x<=a+2 0a-2<-1 a+2<3=>不等式组解集为 -1<=x<=a+2如果a+2<-1 或者a-2>3，若不等式组解集为空集=>a<-3或者a>5。
