\(\left(\frac{5}{7}-x\right)\cdot\frac{14}{5}=\frac{7}{10}+\frac{1}{2}\)

\(\left(\frac{5}{7}-x\right)\cdot\frac{14}{5}=\frac{6}{5}\)

\(\left(\frac{5}{7}-x\right)=\frac{6}{5}\div\frac{14}{5}\)

\(\frac{5}{7}-x=\frac{3}{7}\)

            \(x=\frac{5}{7}-\frac{3}{7}\)

             \(x=\frac{2}{7}\)

\(\left(\frac{5}{7}-x\right).\frac{14}{5}=\frac{7}{10}+\frac{1}{2}\)

<=> \(\left(\frac{5}{7}-x\right).\frac{14}{5}=\frac{6}{5}\)

<=> \(\frac{5}{7}-x=\frac{3}{7}\)

<=> \(x=\frac{2}{7}\)

 ( 5/7 - y ) * 14/5 = 7/10 + 1/2

( 5/7 - y ) x 14/5 = 6/5

 5/7 - y  = 6/5 : 14/5

5/7 - y = 3/7

y = 5/7 - 3/7

y = 2/7

bằng 2/7

2/7 dung 1000000%

Bài 5 :

a) \(\dfrac{y}{4}=\dfrac{9}{y}\)

\(\Rightarrow y^2=36\left(y\ne0\right)\)

\(\Rightarrow y=\pm6\)

b) \(\dfrac{y+7}{20}=\dfrac{5}{y+7}\left(y\ne-7\right)\)

\(\Rightarrow\left(y+7\right)^2=100=10^2\)

\(\Rightarrow\left[{}\begin{matrix}y+7=10\\y+7=-10\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}y=3\\y=-17\end{matrix}\right.\)

c) \(\dfrac{4-5y}{3}=\dfrac{y+2}{5}\)

\(\Rightarrow5\left(4-5y\right)=3\left(y+2\right)\)

\(\Rightarrow20-25y=3y+6\)

\(\Rightarrow28y=14\)

\(\Rightarrow y=\dfrac{14}{28}=\dfrac{1}{2}\)

Bài 4 :

\(\dfrac{a}{5}=\dfrac{b}{7}=\dfrac{c}{10}\)

\(\Rightarrow\dfrac{2a}{10}=\dfrac{3b}{21}=\dfrac{4c}{40}=\dfrac{2a+3b-4c}{10+21-40}=\dfrac{81}{-9}=-9\)

\(\Rightarrow\left\{{}\begin{matrix}a=-9.5=-45\\b=-9.7=-63\\c=-9.10=-90\end{matrix}\right.\)

Đính chính Bài 4 :

\(...\dfrac{2a+3b-4c}{10+21-40}=\dfrac{-81}{-9}=9\)

\(\Rightarrow\left\{{}\begin{matrix}a=9.4=36\\b=9.7=63\\c=9.10=90\end{matrix}\right.\)

(\(\frac{5}{7}-y\)) x \(\frac{14}{5}=\frac{6}{5}\)

\(\frac{5}{7}-y\)            = \(\frac{6}{5}:\frac{14}{5}\)

\(\frac{5}{7}-y\)             = \(\frac{3}{7}\)

            y                 = \(\frac{5}{7}-\frac{3}{7}\)

           y                  = \(\frac{2}{7}\)

\(\left(\frac{5}{7}-y\right)\cdot\frac{14}{5}=\frac{7}{10}+\frac{1}{2}\)

\(\Leftrightarrow\left(\frac{5}{7}-y\right)\cdot\frac{14}{5}=\frac{7}{10}+\frac{5}{10}\)

\(\Leftrightarrow\left(\frac{5}{7}-y\right)\cdot\frac{14}{5}=\frac{6}{5}\)

\(\Leftrightarrow\frac{5}{7}-y=\frac{6}{5}\div\frac{14}{5}\)

\(\Leftrightarrow\frac{5}{7}-y=\frac{3}{7}\)

\(\Rightarrow y=\frac{5}{7}-\frac{3}{7}=\frac{2}{7}\)

Nên tự tính 

Nếu thik thì lấy máy tính tính cho nhanh nha bạn

\(\frac{2}{3}:\frac{y}{9}:\frac{7}{5}=\frac{2}{7}:\frac{3}{5}:\frac{10}{9}\)

\(\frac{2}{3}\cdot\frac{9}{y}\cdot\frac{5}{7}=\frac{2}{7}\cdot\frac{5}{3}\cdot\frac{9}{10}\)

\(\frac{2\cdot9\cdot5}{3\cdot y\cdot7}=\frac{2\cdot5\cdot9}{7\cdot3\cdot10}\)

\(\frac{30}{7\cdot y}=\frac{3}{7}\)

\(\Leftrightarrow30\cdot7=7\cdot y\cdot3\)

210 = 21 * y

y = 10

2/3 : y/9 : 7/5 = 2/7 : 3/5 : 10/9 

= 2/3 : y/9 : 7/5 = 3/70 

= 2/3 : y/9 = 3/70 x 7/5 

= 2/3 : y/9 = 3/50

y/9 = 2/3 : 3/50

y/9 = 100/9 

vậy y = 100 

\(\frac{2}{3}:\frac{y}{9}:\frac{7}{5}=\frac{2}{7}:\frac{3}{5}:\frac{10}{9}\)

\(\frac{2}{3}\times\frac{9}{y}\times\frac{5}{7}=\frac{3}{7}\)

\(\frac{90}{21\times y}=\frac{3}{7}=\frac{90}{210}\)

\(\Rightarrow\frac{90}{21\times y}=\frac{90}{210}\)

=> 21 x y = 210

y = 210 : 21

y= 10

a) \(\frac{7}{10}-y\cdot\frac{3}{4}=\frac{1}{5}\)

\(y\cdot\frac{3}{4}=\frac{7}{10}-\frac{1}{5}\)

\(y\cdot\frac{3}{4}=\frac{1}{2}\)

\(y=\frac{1}{2}:\frac{3}{4}\)

\(y=\frac{1}{2}\cdot\frac{4}{3}\)

\(y=\frac{2}{3}\)

b) \(\frac{5}{6}:\left(y+\frac{7}{9}\right)=\frac{3}{4}\)

\(y+\frac{7}{9}=\frac{5}{6}:\frac{3}{4}\)

\(y+\frac{7}{9}=\frac{5}{6}\cdot\frac{4}{3}\)

\(y+\frac{7}{9}=\frac{10}{9}\)

\(y=\frac{10}{9}-\frac{7}{9}\)

\(y=\frac{3}{9}=\frac{1}{3}\)

**** !

a, \(\frac{7}{10}-y\times\frac{3}{4}=\frac{1}{5}\)

\(\Rightarrow y\times\frac{3}{4}=\frac{7}{10}-\frac{1}{5}\)

\(\Rightarrow y\times\frac{3}{4}=\frac{1}{2}\)

\(\Rightarrow y=\frac{1}{2}\div\frac{3}{4}\)

\(\Rightarrow y=\frac{1}{3}\)

b, \(\frac{5}{6}\div\left(y+\frac{7}{9}\right)=\frac{3}{4}\)

\(\Rightarrow y+\frac{7}{9}=\frac{5}{6}\div\frac{3}{4}\)

\(\Rightarrow y+\frac{7}{9}=\frac{10}{9}\)

\(\Rightarrow y=\frac{10}{9}-\frac{7}{9}\)

\(\Rightarrow y=\frac{1}{3}\)