Proof with auto with typeclass_instances.
intros x b Hb0 Hxb.
destruct (Req_dec x 0) as [Hx0|Hx0].
(* *)
exists 0%R.
split.
now rewrite Rabs_R0.
rewrite Hx0, Rmult_0_l.
apply round_0...
(* *)
specialize (Hxb Hx0).
exists ((round beta fexp rnd x - x) / x)%R.
split. 2: now field.
unfold Rdiv.
rewrite Rabs_mult.
apply Rmult_lt_reg_r with (Rabs x).
now apply Rabs_pos_lt.
rewrite Rmult_assoc, <- Rabs_mult.
rewrite Rinv_l with (1 := Hx0).
now rewrite Rabs_R1, Rmult_1_r.
Qed.

Lemma relative_error_le_conversion :
forall x b, (0 <= b)%R ->
(Rabs (round beta fexp rnd x - x) <= b * Rabs x)%R ->
exists eps,
(Rabs eps <= b)%R /\ round beta fexp rnd x = (x * (1 + eps))%R.