因?yàn)镮f(x)I>1相當(dāng)于f(x)>1或f(x)<-1
[3,+∞]上恒有f(x)>1,則需要f(x)是單調(diào)遞增函數(shù)且滿足f(3)>1即1<a<3
[3,+∞]上恒有f(x)<-1,則需要f(x)是單調(diào)遞減函數(shù)且滿足f(3)<-1即0<a<1/3
因?yàn)镮f(x)I>1相當(dāng)于f(x)>1或f(x)<-1
[3,+∞]上恒有f(x)>1,則需要f(x)是單調(diào)遞增函數(shù)且滿足f(3)>1即1<a<3
[3,+∞]上恒有f(x)<-1,則需要f(x)是單調(diào)遞減函數(shù)且滿足f(3)<-1即0<a<1/3