proposition: Lemma Rinj_Cmult : forall x y : R, Rinj (x * y) = Cmult (Rinj x) (Rinj y).
proof: Proof. intros. unfold Rinj, oneC in |- *. rewrite Cmult_algebrique. RReplace (x * y + - (0 * 0)) (x * y). RReplace (0 * y + x * 0) 0; auto. Qed.