proposition: Lemma cosB x y : cos (x - y) = cos x * cos y + sin x * sin y.
proof: Proof. by rewrite cosD cosN sinN mulrN opprK. Qed.
imports: mathcomp(all_ssreflect ssralg ssrint ssrnum matrix) mathcomp(interval rat) mathcomp(mathcomp_extra boolp classical_sets functions) reals ereal nsatz_realtype signed topology normedtype landau sequences derive realfun exp