Definition thetat : R -> R := theta intr.Definition Rs (t : R) : R :=
  sqrt (Rsqr (xi intr t - xi intr 0) + Rsqr (yi intr t - yi intr 0)).Axiom
  alphas_exists :
    { f : Differential_D2 |
       forall t : R,
       xi intr t = (Rs t * cos (f t) + xi intr 0)%R /\
       yi intr t = (Rs t * sin (f t) + yi intr 0)%R }.Definition alphas := let (a,_) := alphas_exists in a.

Lemma Rs_y : forall t : R, sin (alphas t) <> 0%R -> Rs t = ((yi intr t - yi intr 0) / sin (alphas t))%R.