intro; unfold F in |- *; reg.
unfold Rdiv in |- *;  field.
assert (H0 := rho_pos (vi intr)); red in |- *; intro; rewrite H in H0;
elim (Rlt_irrefl 0 H0).
Qed.

(**********)
Lemma D_F_Rsqr_v :
forall t : R,
(Rsqr (derive_pt F t (F_derivable t)) +
Rsqr (rho (vi intr) / 2) * Rsqr (F t))%R = Rsqr (vi intr).