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(*         *      The Rocq Prover / The Rocq Development Team           *)
(*  v      *         Copyright INRIA, CNRS and contributors             *)
(* <O___,, * (see version control and CREDITS file for authors & dates) *)
(*   \VV/  **************************************************************)
(*    //   *    This file is distributed under the terms of the         *)
(*         *     GNU Lesser General Public License Version 2.1          *)
(*         *     (see LICENSE file for the text of the license)         *)
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Local Open Scope R_scope.
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(** Using series definitions of cos and sin                    *)
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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.

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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)).