For tiny h, f(x+h) = f(x) + hf'(x) ?? Hi all I've been reading about proof of the chain rule and something is making me not sleep at night.. How is that possible that: "for tiny h, f(x+h) = f(x) + hf'(x)" ? Even if 'h' is ultra-small, then "f(x+h)" will always differ from "f(x) + hf'(x)"... I know - the smaller the 'h', the smaller the difference but the difference will always exist for 'h' not equal to zero... So how can we plug this: "f(x) + hf'(x)" instead of "f(x+h)".. Can someone explain this to me?