已知a/b=c/d=e/f=5/7
可得:a=5b/7, c=5d/7 , e=5f/7
所以有:
(a+c-e)/(b+d-f)
=(5b/7+5d/7-5f/7)/(b+d-f)
=5/7(b+d-f)/(b+d-f)
=5/7