log(a/b)[c]=1/{log(c)[a/b]}而：log(c)[a/b]=log(c)[a]－log(c)[b]. =1/{log(a)[c]}－1/{log(b)[c]}. =(1/x1)－(1/x2)则：log(a/b)[c]=1/[(1/x1)－(1/x2)]=(x1x2)/(x2－x1)