proposition: Lemma distance_symm : forall x0 y0 x1 y1:R, dist_euc x0 y0 x1 y1 = dist_euc x1 y1 x0 y0.
proof: Proof. intros x0 y0 x1 y1; unfold dist_euc; apply Rsqr_inj; [ apply sqrt_positivity; apply Rplus_le_le_0_compat | apply sqrt_positivity; apply Rplus_le_le_0_compat | repeat rewrite Rsqr_sqrt; [ unfold Rsqr; ring | apply Rplus_le_le_0_compat | apply Rplus_le_le_0_compat ] ]; apply Rle_0_sqr. Qed.
imports: Rbase Rfunctions SeqSeries Rtrigo1 R_sqrt