a/b=c/d
那么两边减1或加1等式依然成立
则有：a/b-1=c/d-1.①
a/b+1=c/d+1.②
①/②得,(a/b-1)/(a/b+1)=(c/d-1)(c/d+1)
化简可得(a-b)/(a+b)=(c-d)/(c+d)