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_Rsqr :
forall x:R, derive_pt Rsqr x (derivable_pt_Rsqr _) = 2 * x.