Proof with trivial.
intros; apply Rmult_le_compat_l...
left; unfold rho_vi in |- *; apply rho_pos...
Qed.

(* Specifications: *)
Lemma Dyp :
forall t : R,
derive_pt yp t (yp_derivable t) =
(v (vi intr) * sin (thetat intr t - thetat intr 0))%R.