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 

Bài làm:
Ta có: \(\frac{3x}{2.5}+\frac{3x}{5.8}+\frac{3x}{8.11}+\frac{3x}{11.14}=\frac{1}{21}\)

\(\Leftrightarrow x\left(\frac{5-2}{2.5}+\frac{8-5}{5.8}+\frac{11-8}{8.11}+\frac{14-11}{11.14}\right)=\frac{1}{21}\)

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

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

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

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

Học tốt!!!!
 

\(\frac{1}{2}-\frac{1}{14}=\frac{3}{7}\). 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

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

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= 1/21 : 3/7

x= 1/21 . 7/3

x= 1/9

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 . 3/7= 1/21 

=> x = 1/9

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 . 3/7= 1/21 

=> x = 1/9

mình nhé 

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 . 3/7 = 1/21

x = 1/9

\(3x-\frac{15}{5\cdot8}-\frac{15}{8\cdot11}-\frac{15}{11\cdot14}-...-\frac{15}{47\cdot50}=2\frac{1}{10}\)

<=> \(3x-5\left(\frac{3}{5\cdot8}+\frac{3}{8\cdot11}+\frac{3}{11\cdot14}+...+\frac{3}{47\cdot50}\right)=\frac{21}{10}\)

<=> \(3x-5\left(\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+...+\frac{1}{47}-\frac{1}{50}\right)=\frac{21}{10}\)

<=> \(3x-5\left(\frac{1}{5}-\frac{1}{50}\right)=\frac{21}{10}\)

<=> \(3x-5\cdot\frac{9}{50}=\frac{21}{10}\)

<=> \(3x-\frac{9}{10}=\frac{21}{10}\)

<=> \(3x=3\)

<=> \(x=1\)

\(\frac{3}{2.5}\)+\(\frac{3}{5.8}\)+\(\frac{3}{8.11}\)+\(\frac{3}{11.14}\)+\(\frac{3}{14.17}\)

=\(\frac{1}{2}\)-\(\frac{1}{5}\)+\(\frac{1}{5}\)-\(\frac{1}{8}\)+......+\(\frac{1}{14}\)-\(\frac{1}{17}\)

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

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

bn cho mk đi là câu tl sẽ hiện ra

=3(1/2-1/5+1/5-1/8+1/8-1/11+1/11-1/14+1/14-1/17)

=3(1/2-1/17)

=3x15/34

=45/34

mk nha

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

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

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

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

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

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

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

ai k mh mh k lại

k cho mh nha

`(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`

Vậy 

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

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

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

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

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

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

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

\(\frac{4}{2\cdot5}\)+\(\frac{4}{5\cdot8}\)+\(\frac{4}{8\cdot11}\)+.......+\(\frac{4}{8\cdot83}\)=\(\frac{4}{3}\) (\(\frac{3}{2\cdot5}\)+\(\frac{3}{5\cdot8}\) +......+\(\frac{3}{80\cdot83}\) )

=\(\frac{4}{3}\) (\(\frac{1}{2}\) -\(\frac{1}{5}\) +\(\frac{1}{5}\) -\(\frac{1}{8}\) +..........+\(\frac{1}{80}\) -\(\frac{1}{83}\) )

=\(\frac{4}{3}\) (\(\frac{1}{2}\) -\(\frac{1}{83}\) )

=\(\frac{4}{3}\)*\(\frac{81}{166}\) 

=\(\frac{54}{83}\)