Đề bài là tìm x nhé các bạn, các bạn giải giúp mik nhé!!

Bài làm

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

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

\(\Leftrightarrow\frac{x}{2}-\frac{x}{14}=\frac{1}{21}\)

\(\Leftrightarrow\frac{7x}{14}-\frac{x}{14}=\frac{1}{21}\)

\(\Leftrightarrow\frac{6x}{14}=\frac{1}{21}\)

\(\Leftrightarrow126x=14\)

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

Học tôt 

\(\frac{1}{2}-\frac{1}{14}=\frac{3}{7}\). Mà ở đây kết quả là \(\frac{1}{21}\)nên phân số phải nhóm ra ngoài là: 

\(\frac{1}{21}:\frac{3}{7}=\frac{1}{9}\). Ta có:

         \(\frac{1}{9}.\left(\frac{3x}{2.5}+\frac{3x}{5.8}+\frac{3x}{8.11}+\frac{3x}{11.14}\right)=\frac{1}{21}\). Suy ra 3x=9. Vậy x=3

Nhầm nha: Vì có 3x nên phân số nhóm ra ngoài là \(\frac{1}{3}\). Ta có tương tự. Suy ra 3x=3. Vậy x=1

sao 0 có kết quả                                                                          

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

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

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

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

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

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

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

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

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

        \(\Rightarrow x=\frac{1}{9}\)

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

`(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`
`=>x(1/2-1/5+1/5-1/8+1/8-1/11-1/14)=1/21`
`=>x*(1/2-1/14)=1/21`
`=>x*3/7=1/21`
`=>x=1/21:3/7=1/9`
Vậy `x=1/9`

3/(2.5) nhé ghi nhầm

\(\frac{3x}{2\cdot5}+\frac{3x}{5\cdot8}+.........+\frac{3x}{14\cdot17}=\frac{1}{21}\)

\(\Rightarrow3x\cdot\left(\frac{1}{2\cdot5}+\frac{1}{5\cdot7}+........+\frac{1}{14\cdot17}\right)=\frac{1}{21}\)

\(\Rightarrow3x\cdot\left[\frac{1}{3}\left(\frac{3}{2\cdot5}+\frac{3}{5\cdot7}+......+\frac{3}{14\cdot17}\right)\right]=\frac{1}{21}\)

\(\Rightarrow3x\cdot\left[\frac{1}{3}\cdot\left(\frac{1}{2}-\frac{1}{17}\right)\right]=\frac{1}{21}\)

\(\Rightarrow3x\cdot\left(\frac{1}{3}\cdot\frac{15}{34}\right)=\frac{1}{21}\)

\(\Rightarrow3x\cdot\frac{5}{34}=\frac{1}{21}\)\(\Rightarrow x=\frac{1}{21}:\frac{1}{34}:3\)

\(\Rightarrow x=\frac{34}{315}\)

ai k mh mh k lại

k cho mh nha

\(7\frac{x}{2.5}+7\frac{x}{5.8}+.....+7.\frac{x}{17.20}=\frac{21}{10}\)

\(7\left(\frac{x}{2.5}+\frac{x}{5.8}+...+\frac{x}{17.20}\right)=\frac{21}{10}\)

\(\frac{x}{2.5}+\frac{x}{5.8}+...+\frac{x}{17.20}=\frac{21}{70}\)

\(\frac{x.3}{2.5.3}+\frac{x.3}{5.8.3}+...+\frac{x.3}{17.20.3}=\frac{21}{70}\)

\(x.\frac{1}{3}.\left(\frac{3}{2.5}+\frac{3}{5.8}+...+\frac{3}{17.20}\right)=\frac{21}{70}\)

\(x.\frac{1}{3}.\left(\frac{1}{2}-\frac{1}{20}\right)=\frac{21}{70}\)

\(x.\frac{1}{3}.\frac{9}{20}=\frac{21}{70}\)

=> \(x=2\)

\(x=\frac{7x}{2}\)\(-\frac{7x}{5}+\)\(\frac{7x}{5}\)\(-\frac{7x}{8}\)\(+\frac{7x}{8}\)\(-\frac{7x}{11}\)\(+\frac{7x}{11}\)\(-\frac{7x}{14}\)\(+\frac{7x}{14}\)\(-\frac{7x}{17}+\)\(\frac{7x}{17}\)\(-\frac{7x}{20}\)\(=\frac{21}{10}\)

\(x=\frac{7x}{2}\)\(-\frac{7x}{20}\)\(=\frac{21}{10}\)

\(x=\frac{7x.10}{20}\)\(+\frac{7x}{20}\)\(=\frac{21}{10}\)

\(x=\frac{7x.10+7x}{20}\)\(=\frac{21}{10}\)

\(x=\frac{7x.\left(10+2\right)}{20.2}\)\(=\frac{7x.12}{40}\)\(=\frac{21}{10}\)

\(=>\frac{7x.12:4}{40:4}=\)\(\frac{21}{10}\)

\(=>x=1\)