proposition: Lemma cosD x y : cos (x + y) = (cos x * cos y - sin x * sin y).
proof: Proof. by rewrite cos_plus. Qed.
imports: Rdefinitions Raxioms RIneq Rbasic_fun Zwf Epsilon FunctionalExtensionality Ranalysis1 Rsqrt_def Rtrigo1 Reals mathcomp(all_ssreflect ssralg poly mxpoly ssrnum) HB(structures) mathcomp(boolp classical_sets) reals