proposition: Lemma Cos_plus_Pi : forall x : IR, Cos (x[+]Pi) [=] [--] (Cos x).
proof: Proof. intros. transitivity (Cos (x[+]Pi [/]TwoNZ[+]Pi [/]TwoNZ)). apply Cos_wd; rational. eapply eq_transitive. apply Cos_plus_HalfPi. apply un_op_wd_unfolded; apply Sin_plus_HalfPi. Qed.
imports: CoRN.tactics.CornTac CoRN.transc.SinCos