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

Lemma ypt_yp0 : forall t : R, (yp t - yp 0)%R = (- sin (thetat intr 0) * (xi intr t - xi intr 0) + cos (thetat intr 0) * (yi intr t - yi intr 0))%R.