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(* Formalization of the Chou, Gao and Zhang's decision procedure.          *)
(* Julien Narboux (Julien@narboux.fr)                                      *)
(* LIX/INRIA FUTURS 2004-2006                                              *)
(* University of Strasbourg 2008                                           *)
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(** Area elimination *)
(** on_line *)
(** on_line_d *)
(* inter_ll *)
(* on_foot *)
(* on_perp_d *)

Lemma test_area_on_perp_d_1  : forall I X Y A B r,
on_perp_d I X Y  r -> S A B I = S A B X - r / (2 + 2) * (Py X A B - Py Y A B).