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

(**********)
Lemma div_tau_0_Ax2_R :
forall intruder evader : State,
Ax2_R intruder = 0%R -> div_tau intruder evader = 0%R.