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.

Lemma essai18 : derivable_pt (fun x : R => (x + y x)%R) 0.