assert (cmp : 0 < x + sqrt (x ^ 2 + 1)). {
destruct (Rle_dec x 0).
- replace (x ^ 2) with ((-x) ^ 2) by ring.
assert (sqrt ((- x) ^ 2) < sqrt ((-x)^2+1)). {
apply sqrt_lt_1_alt.
split;[apply pow_le | ]; lra.
}
pattern x at 1; replace x with (- (sqrt ((- x) ^ 2))).
+ assert (t:= sqrt_pos ((-x)^2)); lra.
+ simpl; rewrite Rmult_1_r, sqrt_square, Ropp_involutive;[reflexivity | lra].
- apply Rplus_lt_le_0_compat;[apply Rnot_le_gt; assumption | apply sqrt_pos].
}
rewrite exp_ln;[ | assumption].
rewrite exp_Ropp, exp_ln;[ | assumption].
assert (Rmult_minus_distr_r :
forall x y z, (x - y) * z = x * z - y * z) by (intros; ring).
apply Rminus_diag_uniq; unfold Rdiv; rewrite Rmult_minus_distr_r.
assert (t: forall x y z, x - z = y -> x - y - z = 0);[ | apply t; clear t].
- intros a b c H; rewrite <- H; ring.
- apply Rmult_eq_reg_l with (2 * (x + sqrt (x ^ 2 + 1)));[ |
apply Rgt_not_eq, Rmult_lt_0_compat;[apply Rlt_0_2 | assumption]].
field_simplify;[rewrite pow2_sqrt;[field | ] | apply Rgt_not_eq; lra].
apply Rplus_le_le_0_compat;[simpl; rewrite Rmult_1_r; apply (Rle_0_sqr x)|apply Rlt_le, Rlt_0_1].
Qed.

Lemma derivable_pt_lim_arcsinh :
forall x, derivable_pt_lim arcsinh x (/sqrt (x ^ 2 + 1)).