Gợi ý:
1. [tex]\frac{1}{\sqrt{n}+\sqrt{n+1}}=\frac{\sqrt{n+1}-\sqrt{n}}{(\sqrt{n+1}-\sqrt{n})(\sqrt{n+1}+n)}=\frac{\sqrt{n+1}-\sqrt{n}}{1}=\sqrt{n+1}-\sqrt{n}[/tex]
2. [tex]\frac{1}{n\sqrt{n+1}+(n+1)\sqrt{n}}=\frac{1}{\sqrt{n(n+1)}(\sqrt{n}+\sqrt{n+1})}=\frac{\sqrt{n+1}-\sqrt{n}}{\sqrt{n(n+1)}(\sqrt{n+1}+\sqrt{n})(\sqrt{n+1}-\sqrt{n})}=\frac{\sqrt{n+1}-\sqrt{n}}{\sqrt{n(n+1)}}=\frac{1}{\sqrt{n}}-\frac{1}{\sqrt{n+1}}[/tex]