intros; set (f := u eps).
replace (derive_pt (fun y : R => Rsqr (f y)) t (fct_der2 eps t)) with
(derive_pt (fun y : R => (Rsqr (xP y) + (Rsqr (yP y) + Rsqr eps))%R) t
(fct_der3 eps t)).
replace
(derive_pt (fun y : R => (Rsqr (xP y) + (Rsqr (yP y) + Rsqr eps))%R) t
(fct_der3 eps t)) with
(2 * derive_pt xP t (xP_derivable t) * xP t +
2 * derive_pt yP t (yP_derivable t) * yP t)%R.
rewrite (xP_derive t).
rewrite (yP_derive t).
assert (H0 := cond_x intr t).
assert (H1 := cond_y intr t).
cut
(derive_pt (x intr) t (cond_diff (x intr) t) =
derive_pt xi t (xi_derivable t)); [ intro | apply pr_nu ].
cut
(derive_pt (y intr) t (cond_diff (y intr) t) =
derive_pt yi t (yi_derivable t)); [ intro | apply pr_nu ].
rewrite <- H; rewrite <- H2; rewrite H1; rewrite H0.
unfold vi in |- *.
repeat rewrite Rmult_assoc.
rewrite <- Rmult_plus_distr_l.
apply Rmult_le_compat_l.
left; prove_sup0.
rewrite <- Rmult_plus_distr_l.
rewrite (Rmult_comm (u0 t)); apply Rmult_le_compat_l.
left; apply (TypeSpeed_pos (h intr)).
generalize
(sqrt_cauchy (cos (theta intr t)) (sin (theta intr t)) (xP t) (yP t));
rewrite (Rplus_comm (Rsqr (cos (theta intr t))));
rewrite sin2_cos2; rewrite sqrt_1; rewrite Rmult_1_l;
intro H8; unfold u0 in |- *; assumption.
set (H0 := xP_derivable t).
set (H1 := yP_derivable t).
symmetry  in |- *; reg.
unfold derive_pt in |- *; unfold derivable_pt_abs in |- *;
cut
(forall l : R,
derivable_pt_lim (fun y : R => (Rsqr (xP y) + (Rsqr (yP y) + Rsqr eps))%R)
t l <-> derivable_pt_lim (fun y : R => Rsqr (f y)) t l).
intro; unfold projT1 in |- *.
case (fct_der3 eps t); intros.
case (fct_der2 eps t); intros.
assert (H0 := H x).
unfold derivable_pt_abs in d.
elim H0; intros; fold f in d0.
assert (H3 := H1 d).
eapply uniqueness_limite.
apply H3.
apply d0.
intro; unfold derivable_pt_lim in |- *.
split; intros.
elim (H eps0 H0); intros.
exists x; intros; unfold f in |- *; unfold u in |- *.
repeat rewrite Rsqr_sqrt.
repeat rewrite Rplus_assoc.
apply H1; assumption.
repeat apply Rplus_le_le_0_compat; try apply Rle_0_sqr.
repeat apply Rplus_le_le_0_compat; try apply Rle_0_sqr.
elim (H eps0 H0); intros.
exists x; intros; unfold f in H1; unfold u in H1.
replace
((Rsqr (xP (t + h)) + (Rsqr (yP (t + h)) + Rsqr eps) -
(Rsqr (xP t) + (Rsqr (yP t) + Rsqr eps))) / h - l)%R with
((Rsqr (sqrt (Rsqr (xP (t + h)) + Rsqr (yP (t + h)) + Rsqr eps)) -
Rsqr (sqrt (Rsqr (xP t) + Rsqr (yP t) + Rsqr eps))) / h - l)%R.
apply H1; assumption.
repeat rewrite Rsqr_sqrt.
repeat rewrite Rplus_assoc; reflexivity.
repeat apply Rplus_le_le_0_compat; try apply Rle_0_sqr.
repeat apply Rplus_le_le_0_compat; try apply Rle_0_sqr.
Qed.

(**********)
Lemma u_stric_pos : forall (eps : posreal) (t : R), (0 < u eps t)%R.
intros; unfold u in |- *; apply sqrt_lt_R0; apply Rplus_le_lt_0_compat;
[ apply Rplus_le_le_0_compat; apply Rle_0_sqr
| apply Rsqr_pos_lt; red in |- *; intro H; generalize (cond_pos eps);
intro H0; rewrite H in H0; elim (Rlt_irrefl 0 H0) ].