Require Export GeoCoq.Tarski_dev.Annexes.circles.
Require Export GeoCoq.Tarski_dev.Annexes.half_angles.
Require Export GeoCoq.Tarski_dev.Ch12_parallel_inter_dec.
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'.