Section T15.
Context `{T2D:Tarski_2D}.
Context `{TE:@Tarski_euclidean Tn TnEQD}.
(** Lemma 15.2 *)
(** Cong corresponds to length equality.*)
(** Le corresponds to length inequality.*)
(********************************************************)
intros.
unfold Obtuse in H0.
ex_and H0 A'.
ex_and H8 B'.
ex_and H0 C'.
assert(A' <> B' /\ C' <> B' /\ C <> A /\ C' <> B' /\ C <> B).
{
unfold LtA in H8.
spliter.
unfold LeA in H8.
ex_and H8 P.
unfold CongA in H10.
unfold InAngle in H8.
repeat split;
try tauto.
spliter.
auto.
spliter.
auto.
}
spliter.
assert(HH:=l6_11_existence B' C A A' H9 H11).
ex_and HH AA.
assert(HH:=l6_11_existence B' C B C' H12 H13).
ex_and HH CC.
assert(Lt AA CC A B).
{
apply(t18_18 C A B B' AA CC);Cong.
apply lta_comm.
apply(lta_out_lta A' B' C' A C B);
try(apply l6_6; auto).
apply out_trivial; auto.
apply out_trivial; auto.
auto.
}
assert(~Col O E E').
{
unfold Prod in H5.
unfold Ar2 in H5; tauto.
}
assert(exists S1 : Tpoint, Length O E E' AA CC S1).
{
apply(length_existence O E E' AA CC);auto.
}
destruct H20 as [S1 H21].
(* ex_and H20 S1. *)
assert(exists SS : Tpoint, Prod O E E' S1 S1 SS).
{
apply(prod_exists O E E' H19 S1 S1);
unfold Length in H21;tauto.
}
ex_and H20 SS.
apply (per_col _ _ _ CC) in H0;auto.
apply l8_2 in H0.
apply (per_col _ _ _ AA) in H0;auto.
assert(Sum O E E' AC2 BC2 SS).
{
apply(pythagoras O E E' AA CC B' AC BC S1 AC2 BC2 SS H); auto.
apply l8_2; auto.
apply(length_eq_cong_2  O E E' A C AA B'); Cong.
apply(length_eq_cong_2  O E E' B C CC B'); Cong.
}
assert(S2=SS).
{
apply (sum_uniqueness O E E' AC2 BC2);auto.
}
treat_equalities.
assert(HH:= lt_to_ltp O E E' AA CC S1 A B AB H21 H1 H18).
assert(Ps O E S1).
{
assert(AA <> B').
{
intro.
treat_equalities.
unfold Out in H14.
contradiction.
}
assert(CC <> B').
{
intro.
treat_equalities.
unfold Out in H16.
contradiction.
}
assert(AA <> CC).
intro.
treat_equalities.
apply l8_8 in H0.
treat_equalities.
tauto.
apply length_Ps in H21; auto.
intro.
apply sym_equal in H24.
treat_equalities.
apply length_cong in H21.
treat_equalities.
tauto.
}
apply(square_increase_strict O E E' S1 AB S2 AB2); auto.
unfold Length in H1.
unfold Length in H21.
spliter.
repeat split; Col.
apply (length_Ps O E E' A B); auto.
intro.
apply sym_equal in H20.
treat_equalities.
apply ltP_neg in HH.
apply pos_not_neg in H12.
contradiction.
apply out_col in H14; Col.
apply out_col in H16; Col.
Qed.

Lemma pythagoras_obtuse_or_per : forall O E E' A B C AC BC AB AC2 BC2 AB2 S2,
O<>E -> Obtuse A C B \/ Per A C B ->
Length O E E' A B AB -> Length O E E' A C AC -> Length O E E' B C BC ->
Prod O E E' AC AC AC2 -> Prod O E E' BC BC BC2 -> Prod O E E' AB AB AB2 ->
Sum  O E E' AC2 BC2 S2 -> LeP O E E' S2 AB2.