intros f g x a b l axb fg df e ep.
destruct (df e ep) as [d pd].
assert (d'h : 0 < Rmin d (Rmin (b - x) (x - a))).
- apply Rmin_pos;[apply cond_pos | apply Rmin_pos; apply Rlt_0_minus; tauto].
- exists (mkposreal _ d'h); simpl; intros h hn0 cmp.
rewrite <- !fg;[ |assumption | ].
+ apply pd;[assumption |].
apply Rlt_le_trans with (1 := cmp), Rmin_l.
+ assert (-h < x - a).
* apply Rle_lt_trans with (1 := Rle_abs _).
rewrite Rabs_Ropp; apply Rlt_le_trans with (1 := cmp).
rewrite Rmin_assoc; apply Rmin_r.
* assert (h < b - x).
-- apply Rle_lt_trans with (1 := Rle_abs _).
apply Rlt_le_trans with (1 := cmp).
rewrite Rmin_comm, <- Rmin_assoc; apply Rmin_l.
-- split.
++ apply (Rplus_lt_reg_l (- h)).
replace ((-h) + (x + h)) with x by ring.
apply (Rplus_lt_reg_r (- a)).
replace (((-h) + a) + - a) with (-h) by ring.
assumption.
++ apply (Rplus_lt_reg_r (- x)).
replace (x + h + - x) with h by ring.
assumption.
Qed.

Theorem derivable_continuous : forall f, derivable f -> continuity f.