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)再计算代入即可.