log5 3=a log5 4=b
5^a=3,5^b=4
5^(a+b)=3*4
[5^(a+b)]^2=12^2
25^(a+b)=12^2
[25^(a+b)]^(1/2)=(12^2)^(1/2)
25^[(a+b)/2]=12
所以log25 12=1/2(a+b)