提示：用换底公式.
证：
loga(m)/loga(n)
=(lgm/lga)/(lgn/lga)
=lgm/lgn
=logn(m)
logb(m)/logb(n)
=(lgm/lgb)/(lgn/lgb)
=lgm/lgn
=logn(m)
loga(m)/loga(n)=logb(m)/logb(n)