①f(X+Y,Y/X)=(X+Y)^2-Y^2/X^2=X^2+2XY+Y^2-Y^2/X^2,②设A=X+Y，B=Y/X，Y=BX，A=X+BX，X=A/(1+B)，Y=AB/(1+B)，∴f(X，Y)=(X+Y)^2/(1+Y/X)^2-(X+Y)^2(Y/X)/(1+Y/X)^2=X(X+Y)^2(X-Y)/(X^2+Y^2)。
