6. (10 pointe) Let. f : E > R be a function which is three times differentiable on R. Suppose that [f(a:)i 5 1 and ff"'{a:)| E 3 for any a: E R. Prove that if'KI-N E g for any .1: e R. (Hint: Apply Taylor's Theorem to f on [:r w her] and on [33,3: + h] for any a," E R and ii :3: [1.)