(** In a circle the angle at the centre is double of the angle at the circumference. *)

Lemma inscribed_angle_1 : forall O P A B C, A <> B -> B <> C -> A <> C ->
OnCircle A O P -> OnCircle B O P -> OnCircle C O P -> Coplanar A B C O ->
SumA A C B A C B A O B.