f(x)=log3(3x)·log3(x/9)
=(log3(3)+log3(x))*(log3(3^-2)+log3(x))
=2log3(x)-1
设t=log3 (x) (t∈[0,3])
∴f(x)=2t-1 (t∈[0,3])
∴f(x)最大值为5,最小值为-1