2nd derivative proof..help plz? hi, this will be my first post on the forum, although i in the past have looked over it! um, this is NOT a hw problem, but is a problem in my textbook that i attempted to do. it asks to show that if , for a function f, a second derivative exists at x0 to prove that f''(x0) = lim h->0 [f(x0+h)-f(x0-h)-2f(x0)] / h^2 ...At first i thought this would be easy, just using f ' (x0) = limh->0 ( f(x0+h)-f(x0)) / h and f''(x0) = lim (f'(x0+h)-f'(x0))/h but somehow i havent been able to get the expression they ask for??? am i missing something?? (a trick)? thanks!