已知：5A=7B，求（1）（A-B)/B和(2)(A+B)/(A-B)的值

解：依题意B不等于○. A=7B/5
（1）（A－B）/B=(7B/5-B)/B
= B(7/5-1)/B = 2/5
(2) (A+B)/(A-B)
= (7B/5+B)/(7B/5-B)
=B(7/5+1)/B(7/5-1)
=(12/5)/(2/5)
=6

可以吧5A=7B化成A=7B/5
然后（A-B)/B=（7B/5-B)*1/B
=2/5B
（A+B)(A-B)也一样哦

先求出A=7B/5
(1)(A-B)/B
把A=7B/5代入上式
（7B/5-B）/B=B(7/5-1)/B=7/5-1=2/5
(2)(A+B)/(A-B)
=(7B/5+B)/(7B/5-B)
=B(7/5+1)/B(7/5-1)
=12/5 / 2/5
=6