(** We deduce the axioms of the area method. *)
Definition cross_product (u v : vect) : F :=
fst u * snd v - snd u * fst v.
Definition scalar_product (u v : vect) : F :=
fst u * fst v + snd u * snd v.
Definition ratio A B C D :=
let (Ac, _) := coordinates_of_point_F A in
let (Ax, Ay) := Ac in
let (Bc, _) := coordinates_of_point_F B in
let (Bx, By) := Bc in
let (Cc, _) := coordinates_of_point_F C in
let (Cx, Cy) := Cc in
let (Dc, _) := coordinates_of_point_F D in
let (Dx, Dy) := Dc in
let VAB := (Bx-Ax, By-Ay) in
let VCD := (Dx-Cx, Dy-Cy) in
scalar_product VAB VCD / scalar_product VCD VCD.
Definition signed_area A B C :=
let (Ac, _) := coordinates_of_point_F A in
let (Ax, Ay) := Ac in
let (Bc, _) := coordinates_of_point_F B in
let (Bx, By) := Bc in
let (Cc, _) := coordinates_of_point_F C in
let (Cx, Cy) := Cc in
let VAB := (Bx-Ax, By-Ay) in
let VAC := (Cx-Ax, Cy-Ay) in
1 / 2 * cross_product VAB VAC.
(** We introduce this definition to simplify the automatic proofs:*)
Definition twice_signed_area A B C :=
let (Ac, _) := coordinates_of_point_F A in
let (Ax, Ay) := Ac in
let (Bc, _) := coordinates_of_point_F B in
let (Bx, By) := Bc in
let (Cc, _) := coordinates_of_point_F C in
let (Cx, Cy) := Cc in
let VAB := (Bx-Ax, By-Ay) in
let VAC := (Cx-Ax, Cy-Ay) in
cross_product VAB VAC.

Lemma signed_area_sum :
forall A B C D, signed_area A B C =F= signed_area A B D + signed_area A D C + signed_area D B C.