令 X = logA, Y = logB, Z=log(A/B) 。2x = A, 2y = B, 2z = A/B, 则有 2z = A/B = 2x / 2y = 2x-y ，有z = x-y，即 log(A/B) = logA - logB。
log(A/B) = logA -logB
原文：http://www.cnblogs.com/wbb2109/p/5538808.html
