replace (Zabs (radix * Fnum (Fopp p))) with (Zabs (radix * Fnum p));
auto with float.
case p; simpl in |- *; auto with zarith.
intros Fnum1 Fexp1; rewrite <- Zopp_mult_distr_r; apply sym_equal;
apply Zabs_Zopp.
Qed.

Theorem FnormalFabs : forall p : float, Fnormal p -> Fnormal (Fabs p).
intros p; case p; intros a e H; split; auto with float.