(** ** Relation between arccosine, sine and tangent *)

(* Specifications: *)
Lemma sin_acos : forall x, -1 <= x <= 1 ->
sin (acos x) = sqrt (1 - x²).
Lemma tan_acos : forall x, -1 <= x <= 1 ->
tan (acos x) = sqrt (1 - x²) / x.