(y^3 - x^3)/(3.0*y - 3.0*x)
2 2
x y x*y
-- + -- + ---
3 3 3
$$\frac{x^{2}}{3} + \frac{x y}{3} + \frac{y^{2}}{3}$$
2 2
x y x*y
-- + -- + ---
3 3 3
$$\frac{x^{2}}{3} + \frac{x y}{3} + \frac{y^{2}}{3}$$
2 2
x + y + x*y
-------------
3
$$\frac{1}{3} \left(x^{2} + x y + y^{2}\right)$$