proposition: Lemma Derive_inv (f : R -> R) (x : R) : ex_derive f x -> f x <> 0 -> Derive (fun y => / f y) x = - Derive f x / (f x) ^ 2.
proof: Proof. move/Derive_correct => Hf Hl. apply is_derive_unique. by apply is_derive_inv. Qed.
imports: Coq(Reals Psatz ssreflect) Rcomplements Rbar Lim_seq Iter Hierarchy Continuity Equiv