f(x)=[log3(x)-log3(27)][log3(3)+log3(x)]
=[log3(x)-3][log3(x)+1]
令a=log3(x)
1/27<=x<=9
3^(-3)<=x<=3^2
所以-3<=a<=2
y=f(x)=(a-3)(a+1)=a^2-2a-3=(a-1)^2-4
对称轴a=1,开口向上
所以a=1,f(x)最小=-4
a=-3,f(x)最大=12