intros; rewrite Rsx; unfold Rminus, Rdiv in |- *;
rewrite (Rplus_comm (xi intr 0)); rewrite Rplus_assoc;
rewrite Rplus_opp_r; rewrite Rplus_0_r; rewrite Rmult_assoc;
rewrite <- Rinv_r_sym; [ rewrite Rmult_1_r; reflexivity | assumption ].
Qed.

Lemma Rs_y :
forall t : R,
sin (alphas t) <> 0%R -> Rs t = ((yi intr t - yi intr 0) / sin (alphas t))%R.