intros; rewrite Dxp; apply Rmult_le_compat_l;
[ left; apply TypeSpeed_pos
| generalize (theta_inv t H); intro; elim H1; intros;
case (Rcase_abs (thetat intr t - thetat intr 0));
intro;
[ cut (thetat intr 0 - thetat intr t <= rho_vi * t)%R;
[ cut (- (rho_vi * t) <= thetat intr 0 - thetat intr t)%R;
[ intros; rewrite <- (cos_neg (thetat intr t - thetat intr 0));
replace (- (thetat intr t - thetat intr 0))%R with
(thetat intr 0 - thetat intr t)%R;
[ apply cos_decr_1;
[ left;
generalize
(Ropp_lt_gt_contravar (thetat intr t - thetat intr 0) 0 r);
rewrite Ropp_0;
replace (- (thetat intr t - thetat intr 0))%R with
(thetat intr 0 - thetat intr t)%R;
[ intro; assumption | ring ]
| apply Rle_trans with (rho_vi * t)%R; assumption
| apply (rho_t_pos t H)
| assumption
| assumption ]
| ring ]
| apply Rplus_le_reg_l with (thetat intr t + rho_vi * t)%R;
repeat rewrite Rplus_assoc; rewrite Rplus_opp_r;
rewrite Rplus_0_r;
replace
(thetat intr t + (rho_vi * t + (thetat intr 0 - thetat intr t)))%R
with (thetat intr 0 + rho_vi * t)%R;
[ assumption | ring ] ]
| apply Rplus_le_reg_l with (thetat intr t - rho_vi * t)%R;
unfold Rminus in |- *; repeat rewrite Rplus_assoc;
rewrite Rplus_opp_l; rewrite Rplus_0_r; rewrite Rplus_comm;
repeat rewrite Rplus_assoc; rewrite Rplus_opp_l;
rewrite Rplus_0_r; rewrite Rplus_comm; assumption ]
| generalize (Rge_le (thetat intr t - thetat intr 0) 0 r); intro;
cut (thetat intr t - thetat intr 0 <= rho_vi * t)%R;
[ cut (- (rho_vi * t) <= thetat intr t - thetat intr 0)%R;
[ intros; apply cos_decr_1;
[ assumption
| apply Rle_trans with (rho_vi * t)%R; assumption
| apply (rho_t_pos t H)
| assumption
| assumption ]
| apply Rplus_le_reg_l with (thetat intr 0); unfold Rminus in |- *;
rewrite (Rplus_comm (thetat intr t));
repeat rewrite <- Rplus_assoc; rewrite Rplus_opp_r;
rewrite Rplus_0_l; assumption ]
| apply Rplus_le_reg_l with (thetat intr 0); unfold Rminus in |- *;
rewrite (Rplus_comm (thetat intr t)); repeat rewrite <- Rplus_assoc;
rewrite Rplus_opp_r; rewrite Rplus_0_l; assumption ] ] ].
Qed.

Lemma xp0 : xp 0 = (- l * cos (thetat intr 0 + beta))%R.
unfold xp in |- *; rewrite xe_0; rewrite ye_0;
rewrite <- (Rplus_comm (xi intr 0)); unfold Rminus in |- *;
repeat rewrite Ropp_plus_distr; repeat rewrite <- Rplus_assoc;
repeat rewrite Rplus_opp_r; repeat rewrite Rplus_0_l;
rewrite cos_plus; ring.