Definition Alpha (a : R) : bool :=
  let a1 :=
    ((Rsqr (ve evad * T) + Rsqr (l intr evad T) - Rsqr AlertRange) /
     (2 * ve evad * T))%R in
  let a2 := (l intr evad T * cos a)%R in
  match Rle_dec a1 a2 with
  | left _ => true
  | right _ => false
  end.

Lemma Alpha_d_AlertRange_0 : Alpha (beta intr evad T) = true -> (d intr evad <= AlertRange)%R.