(***************************************************************)
(** Using series definitions of cos and sin                    *)
(***************************************************************)

Definition sin_term (a:R) (i:nat) : R :=
  (-1) ^ i * (a ^ (2 * i + 1) / INR (fact (2 * i + 1))).Definition cos_term (a:R) (i:nat) : R :=
  (-1) ^ i * (a ^ (2 * i) / INR (fact (2 * i))).Definition sin_approx (a:R) (n:nat) : R := sum_f_R0 (sin_term a) n.Definition cos_approx (a:R) (n:nat) : R := sum_f_R0 (cos_term a) n.

Lemma pre_cos_bound : forall (a:R) (n:nat), - 2 <= a -> a <= 2 -> cos_approx a (2 * n + 1) <= cos a <= cos_approx a (2 * (n + 1)).