因为If(x)I>1相当于f(x)>1或f(x)<-1
[3,+∞]上恒有f(x)>1,则需要f(x)是单调递增函数且满足f(3)>1即1<a<3
[3,+∞]上恒有f(x)<-1,则需要f(x)是单调递减函数且满足f(3)<-1即0<a<1/3
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