trước tiên ta biến đổi Ùn=$\frac{1}{\sqrt{n+1}+\sqrt{n}}$$\rightarrow U_{n+1}=\frac{1}{\sqrt{n+2}+\sqrt{n+1}}$ xet hieu $U_{n+1}-U_{n}=\frac{1}{\sqrt{n+2}+\sqrt{n+1}}-\frac{1}{\sqrt{n+1}+\sqrt{n}}$NX: $\sqrt{n+2}>\sqrt{n}\rightarrow \sqrt{n+2}+\sqrt{n+1}>\sqrt{n+1}+\sqrt{n}\rightarrow \frac{1}{\sqrt{n+2}+\sqrt{n+1}}<\frac{1}{\sqrt{n+1}+\sqrt{n}}$hAY $U_{n+1}-U_{n}<0\rightarrow $ day giam
xin loi vi may khong danh duoc dau.trước tiên ta biến đổi Ùn=$\frac{1}{\sqrt{n+1}+\sqrt{n}}$$\rightarrow U_{n+1}=\frac{1}{\sqrt{n+2}+\sqrt{n+1}}$ xet hieu $U_{n+1}-U_{n}=\frac{1}{\sqrt{n+2}+\sqrt{n+1}}-\frac{1}{\sqrt{n+1}+\sqrt{n}}$NX: $\sqrt{n+2}>\sqrt{n}\rightarrow \sqrt{n+2}+\sqrt{n+1}>\sqrt{n+1}+\sqrt{n}\rightarrow \frac{1}{\sqrt{n+2}+\sqrt{n+1}}<\frac{1}{\sqrt{n+1}+\sqrt{n}}$hAY $U_{n+1}-U_{n}<0\rightarrow $ day giam
trước tiên ta biến đổi Ùn=$\frac{1}{\sqrt{n+1}+\sqrt{n}}$$\rightarrow U_{n+1}=\frac{1}{\sqrt{n+2}+\sqrt{n+1}}$ xet hieu $U_{n+1}-U_{n}=\frac{1}{\sqrt{n+2}+\sqrt{n+1}}-\frac{1}{\sqrt{n+1}+\sqrt{n}}$NX: $\sqrt{n+2}>\sqrt{n}\rightarrow \sqrt{n+2}+\sqrt{n+1}>\sqrt{n+1}+\sqrt{n}\rightarrow \frac{1}{\sqrt{n+2}+\sqrt{n+1}}<\frac{1}{\sqrt{n+1}+\sqrt{n}}$hAY $U_{n+1}-U_{n}<0\rightarrow $ day giam