gwd-6-35

The operation O is defined by the equation x O y = (x-y)/(x+y), where y dose not equal to -x. If 3 O y = 5 O 4, then y=

A. 1/9

B.4/15

C.5/12

D.12/5

E.15/4

Answer: D

I lost here completely, have no idea at all. Pls help!

思路:X O y = (x-y)/(x+y)>>>3 O y=(3-y)/(3+y);

>>>5 O 4=(5-4)/(5+4)

又 3 O y = 5 O 4>>>(3-y)/(3+y)=(5-4)/(5+4)>>>Y=12/5