(***************************************************************)
(** Radian -> Degree | Degree -> Radian                        *)
(***************************************************************)
Definition toRad (x:R) : R := x * PI * / plat.
Definition toDeg (x:R) : R := x * plat * / PI.

(* Specifications: *)
Lemma rad_deg : forall x:R, toRad (toDeg x) = x.
Lemma toRad_inj : forall x y:R, toRad x = toRad y -> x = y.
Lemma deg_rad : forall x:R, toDeg (toRad x) = x.
Definition sind (x:R) : R := sin (toRad x).