intros; rewrite Rsy; unfold Rminus, Rdiv in |- *;
rewrite (Rplus_comm (yi 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.

(* With the two formulas above, you can prove the following result *)
(* Using the cos_sin_0's lemma                                     *)
Lemma Rs_derivable : derivable Rs.
unfold derivable in |- *; intro.