intros; simpl in |- *; cut (0 < MaxBank)%R.
split.
left; rewrite <- Ropp_0; apply Ropp_lt_gt_contravar; assumption.
left; assumption.
unfold MaxBank in |- *; unfold Rdiv in |- *; apply Rmult_lt_0_compat;
[ prove_sup | apply Rinv_0_lt_compat; prove_sup ].
Qed.

Lemma fct_der1 : forall t : R, derivable_pt (fun t : R => 0%R) t.
intro; reg.