Proof with Geometry.
intros.
apply co_side_elim_1...
Qed.

Lemma elim_length_ratio_inter_ll_1_spec :
forall A C U V P Q Y : Point,
inter_ll Y P Q U V -> S A U V <> 0 -> C <> Y -> parallel A Y C Y ->
A ** Y / C ** Y = S A U V / S C U V.