(7) (4 points) You were given the function f(z, y) = =ry' - ;ry +ry and have already computed that fry(x, y) = 2x y - x + 1. Give a short, computation-free explanation why fyr(x, y) = 21 y = r + 1. Hint. In addition to all results mentioned in the lecture, you may also use that partial derivatives of polynomial functions exist and are themselves polynomial functions