y=lg(x/3)*lg(x/12)-------------题目是不是这样?
y=(lgx-lg3)(lgx-lg12)
lgx=t
y=t^2-(lg3+lg12)t+lg3lg12
y=t^2-lg36t+lg3lg12
y(min)=[4lg3lg12-(lg36)^2]/4
=-(lg2)^2