loga底b×logb底a=1是书上换底公式的结论，log以二为底三的对数乘以log以三为底二的对数等于1
log2(25)*log3(4)=log2(5^2)*log3(2^2)=4log2(5)*log3(2)=4lg5/lg2 * lg2/lg3=4lg5/lg3=4log3(5)
lg25/lg2xlg4/lg3=4lg5/lg3
