1. The problem statement, all variables and given/known data Proff that f´(x) = the lim of delta x approaching to 0 of f (a + delta x) - f (a -delta x) / 2 delta x 2. Relevant equations 3. The attempt at a solution the book hinted me that I should solve it in this way f (a + delta x) - f (a) + f (a) - f (a - delta x) / 2 delta x but im stuck here and I dont know how to proff this. I d appreciate some help, thanks a lot in advance.