assert (H := d_y).
assert (H0 := n_y).
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
apply Rlt_0_1.
red; intro;  rewrite sqrt_1 in H; assert (H0 := R1_neq_R0); elim H0;
assumption.
Qed.
reg.
rewrite sin_0.
rewrite Rsqr_0.
replace (0 + 0 + 1)%R with 1%R; [ apply Rlt_0_1 |  ring ].
Qed.
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
Qed.
reg.
rewrite cos_0.
reflexivity.
Qed.
assert (H := d_y).
reg.
Qed.
assert (H := dy_0).
assert (H0 := d_y).
reg.
Qed.
reg.
apply d_y.
apply d_z.
Qed.

Lemma essai21 : derive_pt (fun x : R => z (y x)) 0 essai20 = 1%R.