Hint Resolve Triangle_axioms_i Triangle_axioms_ii Triangle_axioms_iii.

(* Specifications: *)
Theorem thm4_5ia :
forall (x : Segment) (l : Line),
EqLn l (ln x) -> Incident (origin x) l /\ Incident (extremity x) l.
Theorem thm4_5ib :
forall (x : Segment) (l : Line),
Incident (origin x) l /\ Incident (extremity x) l -> EqLn l (ln x).