(** * Distance *)

Definition dist_euc (x0 y0 x1 y1:R) : R :=
  sqrt (Rsqr (x0 - x1) + Rsqr (y0 - y1)).

Lemma triangle : forall x0 y0 x1 y1 x2 y2:R, dist_euc x0 y0 x1 y1 <= dist_euc x0 y0 x2 y2 + dist_euc x2 y2 x1 y1.