Definition hy (t : R) : R := (r_vi * cos (rho_vi * t))%R.

Lemma ypt_PI2 : forall t : R, (0 <= t)%R -> (rho_vi * t <= PI / 2)%R -> (l * sin (beta + thetat intr 0) + r_vi * (cos (rho_vi * t) - 1) <= yp t)%R /\ (yp t <= l * sin (beta + thetat intr 0) - r_vi * (cos (rho_vi * t) - 1))%R.