(** * Proposition 20 In any triangle two sides taken together in any manner are greater than the remaining one. *)
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Lemma prop_21_2 : forall A B C D A1 A2 D1 D2, OS A B C D -> OS B C A D -> OS A C B D ->
SumS A B A C A1 A2 -> SumS D B D C D1 D2 -> Lt D1 D2 A1 A2.