由a/b=c/d=.=m/n =(a+c+.+m)/(b+d+...+n)
从第二项起 把此c/d换成-c/-d 依次直到m/n=-m/-n
a/b=c/d=.=m/n =(a-c-.-m)/(b-d-.-n)=a/b=c/d=.=m/n