intros a b y x Ha Hb Hy Hx Hxab. cut (x / a * a / y - x / a * (a * b / y) / b <= 1). omega.
apply approx_le1; assumption. Qed.

Lemma approx_cz0 : forall c y z : Z, 0 < c -> 0 < y -> 0 <= z ->
c / y * z / c <= z / y.