Section Inscribed_angle.
Context `{TE:Tarski_euclidean}.
(** The sum of the angles of a triangle is the flat angle. *)

Lemma high_school_exterior_angle_theorem : forall A B C B', A <> B -> B <> C -> A <> C -> A <> B' ->
Bet B A B' -> SumA A B C B C A C A B'.