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(** * Using series definitions of cos and sin                    *)
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Definition sin_ub (a:R) : R := sin_approx a 4.
Definition cos_lb (a:R) : R := cos_approx a 3.
Definition cos_ub (a:R) : R := cos_approx a 4.

Lemma cos_incr_0 :
forall x y:R,
PI <= x ->
x <= 2 * PI -> PI <= y -> y <= 2 * PI -> cos x <= cos y -> x <= y.