tổng quát
$\dfrac{1}{\sqrt[]{n}} = \dfrac{2}{2\sqrt[]{n}}$
+ $\dfrac{2}{2\sqrt[]{n}} < \dfrac{2}{\sqrt[]{n-1}+\sqrt[]{n}} = 2(\sqrt[]{n}-
\sqrt[]{n-1}$
$\Rightarrow A < 2(\sqrt[]{2}-\sqrt[]{1}+\sqrt[]{3}-\sqrt[]{2}+...+\sqrt[]{2025}-
\sqrt[]{2024}$
$\Leftrightarrow A < 2(\sqrt[]{2025}-1)=88$
+$\dfrac{2}{2\sqrt[]{n}} > \dfrac{2}{\sqrt[]{n+1}+\sqrt[]{n}} = 2(\sqrt[]{n+1}-
\sqrt[]{n}$
$\Rightarrow A > 2(\sqrt[]{3}-\sqrt[]{2}+\sqrt[]{4}-\sqrt[]{3}+...+\sqrt[]{2026}-
\sqrt[]{2025}$
$\Leftrightarrow A > 2(\sqrt[]{2026}-\sqrt[]{2}) > 87$
$\Rightarrow 87 < A < 88$
$\Rightarrow A \not\in N$