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