Section T11_2.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.
(** This is exterior angle theorem *)
(** In an isosceles triangle the two base angles are equal. This is Euclid: Book 1, Proposition 5. *)
(** This is Euclid: Book 1, Proposition 18 *)
(** If the base angles are equal, then the triangle is isosceles *)
(** This is Euclid Book I, Proposition 19 *)
(** This is SAS congruence. *)
(** This is ASA congruence. *)
(** This is AAS congruence. *)
(** This is SSS congruence. *)
(** This is SSA congruence with a restriction *)
(** This is SSA congruence with an obtuse angle *)
(** This is SSA congruence with a right angle *)
(** The following lemmas express the triangle inequality only with predicates from Tarski : AC <= AB + BC, and AC = AB + BC <-> Bet A B C *)
(** The following lemmas express the reverse triangle inequality only with predicates from Tarski : BC >= |AC - AB|, and BC = |AC - AB| <-> (A = B \/ A = C \/ Out A B C) *)
(** This is half of Euclid Book I, Proposition 21: if D is inside the triangle ABC then BAC < BDC. *)

Lemma os3__lta : forall A B C D, OS A B C D -> OS B C A D -> OS A C B D ->
LtA B A C B D C.