(** * Definitions on [R] *)

Definition Derive (f : R -> R) (x : R) := real (Lim (fun h => (f (x+h) - f x)/h) 0).

Lemma ex_derive_mult (f g : R -> R) (x : R) : ex_derive f x -> ex_derive g x -> ex_derive (fun x : R => f x * g x) x.