variables

If two distinct non-zero real variables x and y are such that (x + y) is proportional to (x - y) then the value of $\frac{x}{y}$

$$f(x)$$ and $$g(y)$$ are functions of x and y, respectively, and $$f(x)=g(y)$$ for all real values of x and y. Which one of the following options is necessarily TRUE for al x and...