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. *)

Lemma l11_49 : forall A B C A' B' C',
CongA A B C A' B' C' -> Cong B A B' A' -> Cong B C B' C' ->
Cong A C A' C' /\ (A <> C -> CongA B A C B' A' C' /\ CongA B C A B' C' A').