If you find the expression f(u, v)= about u and v, then substitute u=x and v = y.

Now: f(x+y, y/x)=f(u, v)

x？ -Really? =(u/( 1+v))？ -(uv/( 1+v))？ =u？ ( 1-v？ )/( 1+v)？ =u？ ( 1-v)/( 1+v)

So f(u, v)=u? ( 1-v)/( 1+v)

Substitute u=x and v=y, and you get it.

f(x，y)=x？ ( 1-y)/( 1+y)