换底公式：loga b=lgb/lga
log2 25×log3 4×log5 9
=(lg25/lg2)×(lg4/lg3)×(lg9/lg5)
=(2lg5/lg2)×(2lg2/lg3)×(2lg3/lg5)
=8
证明：
loga b×logb c ×logc a
=(lgb/lga)×(lgc/lgb)×(lga/lgc)
=(lga/lga)×(lgb/lgb)×(lgc/lgc)
=1