f(x)=(lgx-lga)(lgx-lgb)=(lgx)^2-(lga+lgb)lgx+lga*lgb
f(x)min=[4lgalgb-(lgab)^2]/4=-1/4
4lgalgb-(lgab)^2=-1
8lga-(lga+2)^2=-1
(lga)^2-4lga+3=0
(lga-1)(lga-3)=0
a=10或a=1000
logaB=2或2/3