log(5)3=a,log(5)4=b
log(3)5=1/a,log(3)25=2a,
log(4)5=1/b,log(4)25=2/b
log(25)3=a/2
log(25)4=b/2
log(25)12
=log(25)3*4
=log(25)3+log(25)4
=(a+b)/2