intros f x l a df;
apply (derivable_pt_lim_ext (fun y => /a * f y)).
- intros z; rewrite Rmult_comm; reflexivity.
- unfold Rdiv; rewrite Rmult_comm; apply derivable_pt_lim_scal; assumption.
Qed.

(* Specifications: *)
Lemma derivable_pt_lim_scal_right :
forall f x l a, derivable_pt_lim f x l ->
derivable_pt_lim (fun y => f y * a) x (l * a).