Math question functions help me? I have to find find $f(x,y)$ that satisfies 
\begin{align}
f(x+y,x-y) &= xy + y^2 \\
f(x+y, \frac{y}x ) &= x^2 - y^2
\end{align}
So I first though about replacing $x+y=X$ and $x-y=Y$ in the first one but then what?

Then express $x,y$ in terms of the new $X,Y$, and you're just there..
Similarly for the second question, $X:=x+y,\ \ Y:=y/x$, then $X/x=Y+1$, which leads to $x$ in terms of $X,Y$...
