So, using the definition of derivative ---
f'(x) = lim h->0 f (x+h) - f(x)/h, if we have h = y-x then as you said from the growth condition, then the numerator which is f(y) - f(x) is much smaller than the denominator y-x, so f' = 0 for y-x close to 0. Is that right?
Also, when I'm changing h...