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(** Radian -> Degree | Degree -> Radian                        *)
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Definition plat : R := 180.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.