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 *)

Lemma l8_21_3 : forall A B C D X, Coplanar A B C D -> ~ Coplanar A B C X ->
exists P T, Orth A B C D P /\ Coplanar A B C T /\ Bet X T P.