intros f a b l u ri ab intf.
rewrite <- !(fun l => RiemannInt_P15 (RiemannInt_P14 a b l)).
split; apply RiemannInt_P19; try assumption;
intros x intx; unfold fct_cte; destruct (intf x intx); assumption.
Qed.

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