proposition: Theorem sub_firq : forall a b c : R, forall zi : FF, NZR (c - b) -> BND ((a - c) / (c - b)) zi -> BND (((a - b) - (c - b)) / (c - b)) zi.
proof: Proof. intros a b c zi Hac Hz. replace (((a - b) - (c - b)) / (c - b))%R with ((a - c) / (c - b))%R. exact Hz. field. exact Hac. Qed.
imports: Reals Gappa_common