proposition: Lemma l15_3 : forall O E E' A B C, Sum O E E' A B C -> Cong O B A C.
proof: Proof. intros. assert(Ar2 O E E' A B C). unfold Sum in H. spliter. assumption. unfold Ar2 in H0. spliter. induction (eq_dec_points A O). subst A. assert(B = C). apply (sum_uniqueness O E E' O B); auto. apply sum_O_B; auto. subst C. Cong. apply sum_cong in H; auto. unfold Parallelogram_flat in H. spliter. Cong. Qed.
imports: GeoCoq.Tarski_dev.Ch14_order