Cách 1:

Ta có: \({y_n} = \sqrt {n + 1}  - \sqrt n  = \frac{{\left( {\sqrt {n + 1}  - \sqrt n } \right)\left( {\sqrt {n + 1}  + \sqrt n } \right)}}{{\sqrt {n + 1}  + \sqrt n }} = \frac{{\left( {n + 1} \right) - n}}{{\sqrt {n + 1}  + \sqrt n }} = \frac{1}{{\sqrt {n + 1}  + \sqrt n }}\)

\( \Rightarrow {y_{n + 1}} = \frac{1}{{\sqrt {\left( {n + 1} \right) + 1}  - \sqrt {n + 1} }} = \frac{1}{{\sqrt {n + 2}  + \sqrt {n + 1} }}\)

Xét hiệu:

\(\begin{array}{l}{y_{n + 1}} - {y_n} = \frac{1}{{\sqrt {n + 2}  + \sqrt {n + 1} }} - \frac{1}{{\sqrt {n + 1}  + \sqrt n }} = \frac{{\left( {\sqrt {n + 1}  + \sqrt n } \right) - \left( {\sqrt {n + 2}  + \sqrt {n + 1} } \right)}}{{\left( {\sqrt {n + 2}  + \sqrt {n + 1} } \right)\left( {\sqrt {n + 1}  + \sqrt n } \right)}}\\ = \frac{{\sqrt {n + 1}  + \sqrt n  - \sqrt {n + 2}  - \sqrt {n + 1} }}{{\left( {\sqrt {n + 2}  + \sqrt {n + 1} } \right)\left( {\sqrt {n + 1}  + \sqrt n } \right)}} = \frac{{\sqrt n  - \sqrt {n + 2} }}{{\left( {\sqrt {n + 2}  + \sqrt {n + 1} } \right)\left( {\sqrt {n + 1}  + \sqrt n } \right)}}\end{array}\)

\(\forall n \in {\mathbb{N}^*}\) ta có:

\(\begin{array}{l}\left. \begin{array}{l}0 < n < n + 2 \Leftrightarrow \sqrt n  < \sqrt {n + 2}  \Leftrightarrow \sqrt n  - \sqrt {n + 2}  < 0\\\sqrt {n + 2}  > 0,\sqrt {n + 1}  > 0,\sqrt n  > 0 \Leftrightarrow \left( {\sqrt {n + 2}  + \sqrt {n + 1} } \right)\left( {\sqrt {n + 1}  + \sqrt n } \right) > 0\end{array} \right\}\\ \Rightarrow \frac{{\sqrt n  - \sqrt {n + 2} }}{{\left( {\sqrt {n + 2}  + \sqrt {n + 1} } \right)\left( {\sqrt {n + 1}  + \sqrt n } \right)}} < 0\end{array}\)

Vậy \({y_{n + 1}} - {y_n} < 0 \Leftrightarrow {y_{n + 1}} < {y_n}\). Vậy dãy số \(\left( {{y_n}} \right)\) là dãy số giảm.

Cách 2:

Ta có: \({y_n} = \sqrt {n + 1}  - \sqrt n  = \frac{{\left( {\sqrt {n + 1}  - \sqrt n } \right)\left( {\sqrt {n + 1}  + \sqrt n } \right)}}{{\sqrt {n + 1}  + \sqrt n }} = \frac{{\left( {n + 1} \right) - n}}{{\sqrt {n + 1}  + \sqrt n }} = \frac{1}{{\sqrt {n + 1}  + \sqrt n }}\)

\( \Rightarrow {y_{n + 1}} = \frac{1}{{\sqrt {\left( {n + 1} \right) + 1}  - \sqrt {n + 1} }} = \frac{1}{{\sqrt {n + 2}  + \sqrt {n + 1} }}\)

\(\forall n \in {\mathbb{N}^*}\) ta có:

\(\begin{array}{l}0 < n < n + 2 \Leftrightarrow \sqrt n  < \sqrt {n + 2}  \Leftrightarrow \sqrt {n + 1}  + \sqrt n  < \sqrt {n + 2}  + \sqrt {n + 1} \\ \Leftrightarrow \frac{1}{{\sqrt {n + 1}  + \sqrt n }} > \frac{1}{{\sqrt {n + 2}  + \sqrt {n + 1} }} \Leftrightarrow {y_n} > {y_{n + 1}}\end{array}\)

Vậy dãy số \(\left( {{y_n}} \right)\) là dãy số giảm.

\(u_{n+1}=\dfrac{3^{n+1}-1}{2^{n+1}}=\dfrac{3\cdot3^n-1}{2\cdot2^n}\)

Ta có: 

\(u_{n+1}-u_n=\dfrac{3\cdot3^n-1}{2\cdot2^n}-\dfrac{3^n-1}{2^n}=\dfrac{3\cdot3^n-1-2\cdot3^n+2}{2\cdot2^n}=\dfrac{3^n+1}{2^{n+1}}>0\forall x\in N\)*

Do đó, \(u_{n+1}-u_n>0\Leftrightarrow u_{n+1}>u_n\)

Vậy dãy số \(\left(u_n\right)\) là dãy số tăng.

\(u_n=\dfrac{1}{n+1}\Rightarrow u_{n+1}=\dfrac{1}{n+2}\)

\(\Rightarrow u_n-u_{n+1}=\dfrac{1}{n+1}-\dfrac{1}{n+2}=\dfrac{1}{\left(n+1\right)\left(n+2\right)}>0\)

\(\Rightarrow u_{n+1}< u_n\Rightarrow\) dãy giảm

Do \(\dfrac{1}{n+1}>0\Rightarrow\) dãy bị chặn dưới bởi 0

\(u_n-1=\dfrac{1}{n+1}-1=-\dfrac{n}{n+1}< 0\Rightarrow u_n< 1\)

\(\Rightarrow\) Dãy bị chặn trên bởi 1

\(\Rightarrow\) Dãy bị chặn

\(u_n=\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+...+\dfrac{1}{n\left(n+1\right)}\)

\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{n}-\dfrac{1}{n+1}\)

\(=1-\dfrac{1}{n+1}< 1\)

=>Hàm số bị chặn trên tại \(u_n=1\)

\(n+1>=1\)

=>\(\dfrac{1}{n+1}< =1\)

=>\(-\dfrac{1}{n+1}>=-1\)

=>\(1-\dfrac{1}{n+1}>=-1+1=0\)

=>Hàm số bị chặn dưới tại 0

\(u_n=1-\dfrac{1}{n+1}=\dfrac{n+1-1}{n+1}=\dfrac{n}{n+1}\)

\(\dfrac{u_n}{u_{n+1}}=\dfrac{n}{n+1}:\dfrac{n+1}{n+2}=\dfrac{n^2+2n}{n^2+2n+1}< 1\)

=>(un) là dãy số tăng

Ta có: \({u_{n + 1}} = \frac{1}{{n + 1 + 1}} = \frac{1}{{n + 2}}\).

Mà \(\left( {n + 2} \right) > \left( {n + 1} \right)\) suy ra \(\frac{1}{{n + 2}} < \frac{1}{{n + 1}}\).

Tức là \({u_{n + 1}} < {u_n},\;\forall n \in {N^*}\).

Vậy \(\left( {{u_n}} \right)\) là dãy số giảm.

Chọn A

\(u_n=\dfrac{n+2}{n}\)

\(u_{n+1}=\dfrac{n+3}{n+1}\)

\(\Rightarrow u_{n+1}-u_n=\dfrac{n+3}{n+1}-\dfrac{n+2}{n}\)

\(\Rightarrow u_{n+1}-u_n=\dfrac{n\left(n+3\right)-\left(n+1\right)\left(n+2\right)}{n\left(n+1\right)}\)

\(\Rightarrow u_{n+1}-u_n=\dfrac{n^2+3n-\left(n^2+3n+2\right)}{n\left(n+1\right)}\)

\(\Rightarrow u_{n+1}-u_n=\dfrac{n^2+3n-n^2-3n-2}{n\left(n+1\right)}\)

\(\Rightarrow u_{n+1}-u_n=\dfrac{-2}{n\left(n+1\right)}< 0\)

Vậy dãy số \(u_n\) đã cho là dãy giảm

\(u_n=\sqrt[]{n+10}-\sqrt[]{n+2}\)

\(\Leftrightarrow u_n=\dfrac{n+10-\left(n+2\right)}{\sqrt[]{n+10}+\sqrt[]{n+2}}\)

\(\Leftrightarrow u_n=\dfrac{8}{\sqrt[]{n+10}+\sqrt[]{n+2}}\)

\(u_{n+1}=\sqrt[]{n+11}-\sqrt[]{n+3}\)

\(\Leftrightarrow u_{n+1}=\dfrac{n+11-\left(n+3\right)}{\sqrt[]{n+11}+\sqrt[]{n+3}}\)

\(\Leftrightarrow u_{n+1}=\dfrac{8}{\sqrt[]{n+11}+\sqrt[]{n+3}}\)

\(u_{n+1}-u_n=8\left(\dfrac{1}{\sqrt[]{n+11}+\sqrt[]{n+3}}-\dfrac{1}{\sqrt[]{n+10}+\sqrt[]{n+2}}\right)\)

mà \(\dfrac{1}{\sqrt[]{n+11}+\sqrt[]{n+3}}< \dfrac{1}{\sqrt[]{n+10}+\sqrt[]{n+2}}\)

\(\Rightarrow u_{n+1}-u_n< 0\)

Vậy dãy đã cho là dãy số giảm

\(u_n=\dfrac{3^n-1}{2^n}\)

\(\Rightarrow u_{n+1}=\dfrac{3^{n+1}-1}{2^{n+1}}\)

\(\Rightarrow u_{n+1}-u_n=\dfrac{3^{n+1}-1}{2^{n+1}}-\dfrac{3^n-1}{2^n}\)

\(\Rightarrow u_{n+1}-u_n=\dfrac{2^n.3^{n+1}-2^n-2^{n+1}.3^n+2^{n+1}}{2^n.2^{n+1}}\)

\(=\dfrac{2^n.3^n\left(3-2\right)-2^n\left(2-1\right)}{2^{2n+1}}\)

\(=\dfrac{2^n.\left(3^n-1\right)}{2^{2n+1}}\)

\(=\dfrac{\left(3^n-1\right)}{2}>0\left(n>1\right)\)

Vậy dãy \(u_n\)đã cho tăng