unfold div_tau, Ax2_R in |- *; intros; unfold dxdt, dydt in |- *;
replace (intruderSpeed * cosd (heading intruder) - evaderSpeed)%R with
(- evaderSpeed + cosd (heading intruder) * intruderSpeed)%R;
[ rewrite <- (Rmult_comm (sind (heading intruder))); rewrite Rplus_comm;
assumption
| ring ].
Qed.

(**********)
Lemma R_Rmin :
forall (intruder evader : State) (t : R),
div_tau intruder evader <> 0%R ->
Rsqr (RR intruder evader t) =
(Ax2_R intruder * Rsqr (t - tmin intruder evader) +
Rsqr_Rmin intruder evader)%R.