Definition hx (t : R) : R := (r_vi * sin (rho_vi * t))%R.

Lemma xpt_xp0 : forall t : R, (xp t - xp 0)%R = (cos (thetat intr 0) * (xi intr t - xi intr 0) + sin (thetat intr 0) * (yi intr t - yi intr 0))%R.