Suppose that a function f(x, y) is continuous on xy-plane (R^2), f(x, y) > 0 when x^2 + y^2 not equal to 0, and f(cx, cy) = c2f(x, y), c > 0, (x, y) R^2, prove that a and b (0 < a b), such that a(x^2 + y^2) f(x, y) b(x^2 + y^2), (x, y) R^2