由b分之a=d分之c=f分之e=7分之5
得①5b=7a,a=5b/7
②5d=7c, c=5d/7
③5f=7e,e=5f/7
(1)(a-c)/(b-d)
=(5b/7-5d/7)/(b-d)
=5/7(b-d)/(b-d)
=5/7
（2）(2a-c+3e)/(2b-d+3f)
=(2*5b/7-5d/7+3*5f/7)/(2b-d+3f)
=5/7(2b-d+3f)/(2b-d+3f)
=5/7