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.

Lemma derive_pt_opp_rev :
forall f (x:R) (pr1:derivable_pt (- f) x),
derive_pt (- f) x pr1 = - derive_pt f x (derivable_pt_opp_rev _ _ pr1).