unfold alphas in |- *; case alphas_exists; trivial.
Qed.

(* Specifications: *)
(**********)
Lemma Rsx : forall t : R, xi intr t = (xi intr 0 + Rs t * cos (alphas t))%R.
intros; generalize (alphas_def t); intro H; elim H; intros;
rewrite Rplus_comm; assumption.