<image>Given the function f(x), for any x, y ∈ R, it is always true that f(x) + f(y) = f(x + y). Also, when x > 0, f(x) < 0, and f(1) = -. 

(1) Prove that f(x) is a decreasing function on R. 
(2) Find the maximum and minimum values of f(x) on the interval [-3, 3].