(** Note: tan_sin_Rabs wouldn't make a lot of sense, because one would need Rabs on both sides *)

(* Specifications: *)
Lemma tan_cos : forall x, 0 <= sin x ->
tan x = sqrt (1 - (cos x)²) / cos x.
Lemma tan_cos_opp : forall x, 0 >= sin x ->
tan x = - sqrt (1 - (cos x)²) / cos x.