proposition: Lemma sinD x y : sin (x + y) = sin x * cos y + cos x * sin y.
proof: Proof. have /eqP := sinD_cosD x y. rewrite paddr_eq0 => [/andP[]||]; try exact: sqr_ge0. by rewrite sqrf_eq0 subr_eq0 => /eqP. 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