intros.
apply Zgt_trans with (m := (n + p)%Z).
rewrite Zplus_comm.
rewrite Zplus_comm with (n := n).
apply Zplus_gt_compat_l.
assumption.
apply Zplus_gt_compat_l.
assumption.
Qed.

Lemma square_pos : forall a : Z, a <> 0%Z -> (0 < a * a)%Z.
Hint Resolve square_pos: zarith.