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1 tháng 5

\(\dfrac{3x}{2.5}+\dfrac{3x}{5.8}+\dfrac{3x}{8.11}+\dfrac{3x}{11.14}=\dfrac{1}{21}\)

\(3x.\left(\dfrac{1}{2.5}+\dfrac{1}{5.8}+\dfrac{1}{8.11}+\dfrac{1}{11.14}\right)=\dfrac{1}{21}\)

\(3x\left(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{14}\right)=\dfrac{1}{21}\)

\(3x.\left(\dfrac{1}{2}-\dfrac{1}{14}\right)\)                                                  \(=\dfrac{1}{21}\)

\(3x.\dfrac{3}{7}\)                                                                 \(=\dfrac{1}{21}\)

\(3x\)                                                                     \(=\dfrac{1}{21}:\dfrac{3}{7}\)

\(3x=\dfrac{1}{9}\)

\(x=\dfrac{1}{9}:3\)

\(x=\dfrac{1}{27}\)

1 tháng 5

1

x(3/2.5 + 3/5.8 + 3/8.11 + 3/11.14)=1/21

x(1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + 1/11- 1/14)=1/21

x(1/2 - 1/14)=1/21

x . 6/14=1/21

x=1/21 : 6/14=1/21 . 14/6=2/3

4 tháng 4 2022

\(x\) \((\)\(\dfrac{3}{2.5}\) \(+ \) \(\dfrac{3}{5.8}\) \(+\) \(\dfrac{3}{8.11}\) \(+\) \(\dfrac{3}{11.14}\)\()\) \(=\) \(\dfrac{1}{21}\)
\(x\) \((\)\(\dfrac{1}{2}\) \(-\) \(\dfrac{1}{5}\) \(+\) \(\dfrac{1}{5}\) \(-\) \(\dfrac{1}{8}\) \(+\) \(\dfrac{1}{8}\) \(-\) \(\dfrac{1}{11}\) \(+\) \(\dfrac{1}{11}\) \(-\) \(\dfrac{1}{14}\)\()\) \(=\) \(\dfrac{1}{21}\)
\(x\) \((\)\(\dfrac{1}{2}\) \(-\) \(\dfrac{1}{14}\)\()\) \(=\) \(\dfrac{1}{21}\)
\(x\) x \(\dfrac{3}{7}\) \(=\) \(\dfrac{1}{21}\)
\(x\)        \(=\) \(\dfrac{1}{21}\) \(:\) \(\dfrac{3}{7}\) 
\(x\)        \(=\) \(\dfrac{1}{9}\)

 

30 tháng 8 2017

a, dễ, tự làm

b, \(\dfrac{3x}{2.5}+\dfrac{3x}{5.8}+.........+\dfrac{3x}{11.14}=\dfrac{1}{21}\)

\(\Leftrightarrow x\left(\dfrac{3}{2.5}+\dfrac{3}{5.8}+.........+\dfrac{3}{11.14}\right)=\dfrac{1}{21}\)

\(\Leftrightarrow x\left(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+.....+\dfrac{1}{11}-\dfrac{1}{14}\right)=\dfrac{1}{21}\)

\(\Leftrightarrow x\left(\dfrac{1}{2}-\dfrac{1}{14}\right)=\dfrac{1}{21}\)

\(\Leftrightarrow x.\dfrac{3}{7}=\dfrac{1}{21}\)

\(\Leftrightarrow x=\dfrac{1}{9}\)

Vậy ...

30 tháng 8 2017

a) (x-2)3 = (x-2)2

<=> (x-2)3-(x-2)2 = 0

<=> (x-2)2(x-2-1) = 0

<=> \(\left\{{}\begin{matrix}\left(x-2\right)^2=0\\x-3=0\end{matrix}\right.\)

<=> \(\left\{{}\begin{matrix}x=2\\x=3\end{matrix}\right.\)

b) \(\dfrac{3x}{2.5}+\dfrac{3x}{5.8}+...+\dfrac{3x}{11.14}=\dfrac{1}{21}\)

<=> \(x\left(\dfrac{3}{2.5}+\dfrac{3}{5.8}+...+\dfrac{3}{11.14}\right)=\dfrac{1}{21}\)

<=> \(x\left(\dfrac{1}{2}-\dfrac{1}{14}\right)=\dfrac{1}{21}\)

<=> \(x=\dfrac{1}{21}:\dfrac{3}{7}\)

<=> \(x=\dfrac{1}{9}\)

15 tháng 7 2023

`3x-15/(5*8)-15/(8*11)-15/(11*14)-...-15/(47*50)=2 1/10`

`3x-(15/(5*8)+15/(8*11)+15/(11*14)+...+15/(47*50))=21/10`

`3x-5(3/(5*8)+3/(8*11)+3/(11*14)+...+3/(47*50))=21/10`

`3x-5(1/5-1/8+1/8-1/11+1/11-1/14+...+1/47-1/50)=21/10`

`3x-5(1/5-1/50)=21/10`

`3x-5*9/50=21/10`

`3x-9/10=21/10`

`3x=21/10+9/10`

`3x=3`

`x=1`

14 tháng 5 2017

\(\dfrac{3x}{2.5}+\dfrac{3x}{5.8}+\dfrac{3x}{8.11}+\dfrac{3x}{11.14}=\dfrac{1}{21}\)

\(\Rightarrow x\left(\dfrac{3}{2.5}+\dfrac{3}{5.8}+\dfrac{3}{8.11}+\dfrac{3}{11.14}\right)=\dfrac{1}{21}\)

\(\Rightarrow x\left(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{14}\right)=\dfrac{1}{21}\)

\(\Rightarrow x\left(\dfrac{1}{2}-\dfrac{1}{14}\right)=\dfrac{1}{21}\)

\(\Rightarrow x.\dfrac{3}{7}=\dfrac{1}{21}\)

\(\Rightarrow x=\dfrac{1}{21}.\dfrac{7}{3}\)

\(\Rightarrow x=\dfrac{1}{9}\)

Vậy \(x=\dfrac{1}{9}\)

14 tháng 5 2017

cậu giỏi thật đấy!

1 tháng 4 2023

\(\dfrac{3}{2\cdot5}+\dfrac{3}{5\cdot8}+\dfrac{3}{8\cdot11}+\dfrac{3}{11\cdot14}+\dfrac{3}{14\cdot17}\)

\(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{14}+\dfrac{1}{14}-\dfrac{1}{17}\)

\(=\dfrac{1}{2}-\dfrac{1}{17}\)

\(=\dfrac{15}{34}\)

Vì \(\dfrac{15}{34}< \dfrac{1}{2}=>\dfrac{3}{2\cdot5}+\dfrac{3}{5\cdot8}+\dfrac{3}{8\cdot11}+\dfrac{3}{11\cdot14}+\dfrac{3}{14\cdot27}< \dfrac{1}{2}\)

12 tháng 5 2015

sao 0 có kết quả                                                                          

12 tháng 5 2015

3x/2.5 + 3x/5.8+3x/8.11+3x/11.14 = 1/21

 x(1/2-1/5+1/5-1/8+1/8-1/11+1/11-1/14) = 1/21

 x(1/2-1/14) = 1/21

x . 3/7 = 1/21

=> x = 1/21 : 3/7 

=> x = 1/9

Hihi mình giải zầy mk hk bik đúng hay sai

 

12 tháng 5 2015

Ta có : \(\frac{3x}{2\times5}+\frac{3x}{5\times8}+\frac{3x}{8\times11}+\frac{3x}{11\times14}=\frac{1}{21}\)

    \(\Rightarrow x\times\left(\frac{3}{2\times5}+\frac{3}{5\times8}+\frac{3}{8\times11}+\frac{3}{11\times14}\right)=\frac{1}{21}\)

    \(\Rightarrow x\times\left(\frac{1}{2\times5}+\frac{1}{5\times8}+\frac{1}{8\times11}+\frac{1}{11\times14}\right)=\frac{1}{21}\)

         \(x\times\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}\right)=\frac{1}{21}\) 

         \(x\times\left(\frac{1}{2}-\frac{1}{14}\right)\)                                          \(=\frac{1}{21}\)

        \(x\times\frac{3}{7}\)                                                             \(=\frac{1}{21}\)

        \(x\)                                                                        \(=\frac{1}{21}\div\frac{3}{7}=\frac{1}{21}\times\frac{7}{3}\)

        \(\Rightarrow x=\frac{1}{9}\)

6 tháng 5 2018

Ta có 3x/2.5+3x/5.8+3x/8.11+3x/11.14=1/21

=>x(3/2.5+3/5.8+3/8.11+3/11.14)=1/21

=>3x(1/2.5+1/5.8+1/8.11+1/11.14)=1/21

=>3x(1/2-1/14)=1/21

=>3x.3/7=1/21

=>3x=1/21:3/7

=>3x=1

=>x=1:3=1/3

14 tháng 7 2017

\(\dfrac{1}{5\cdot8}+\dfrac{1}{8\cdot11}+\dfrac{1}{11\cdot14}+...+\dfrac{1}{x\left(x+3\right)}=\dfrac{101}{1540}\)

\(\Leftrightarrow\dfrac{1}{3}\left(\dfrac{3}{5\cdot8}+\dfrac{3}{8\cdot11}+\dfrac{3}{11\cdot14}+...+\dfrac{3}{x\left(x+3\right)}\right)=\dfrac{101}{1540}\)

\(\Leftrightarrow\dfrac{1}{3}\left(\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+...+\dfrac{1}{x}-\dfrac{1}{x+3}\right)=\dfrac{101}{1540}\)

\(\Leftrightarrow\dfrac{1}{3}\left(\dfrac{1}{5}-\dfrac{1}{x+3}\right)=\dfrac{101}{1540}\)\(\Leftrightarrow\dfrac{1}{5}-\dfrac{1}{x+3}=\dfrac{303}{1540}\)

\(\Leftrightarrow\dfrac{1}{x+3}=\dfrac{1}{308}\)\(\Rightarrow x+3=308\Rightarrow x=305\)