intros f a b k ri.
apply Riemann_integrable_ext with
(f := fun x => 0 + k * f x).
{ intros; ring. }
apply (RiemannInt_P10 _ (RiemannInt_P14 _ _ 0) ri).
Qed.
Arguments Riemann_integrable_scal [f a b] k _ eps.

Lemma Riemann_integrable_Ropp :
forall f a b, Riemann_integrable f a b ->
Riemann_integrable (fun x => - f x) a b.