proposition: Lemma neg_cos : forall x:R, cos (x + PI) = - cos x.
proof: Proof. intro x; rewrite cos_plus; rewrite sin_PI; rewrite cos_PI; ring. Qed.
imports: Rbase Rfunctions SeqSeries Rtrigo_fun Rtrigo_def Rtrigo_alt Cos_rel Cos_plus ZArith_base Zcomplements Lia Lra Ranalysis1 Rsqrt_def PSeries_reg