a,{ [ - 2/3 + 3/7 ] + [ -1/3 + 4/7 ] } ; 4/5 =

   (   -3/3 + 7/7 ) : 4/5 = 

(-1+1 ) :4/5 = 

0 :4/5 = 0

-5/21 : 4 / 5 + 5/21 : 4/5 = -25 / 84 + 25 / 84 = 0 

****

\(\dfrac{2}{5}+\dfrac{3}{7}+\dfrac{3}{5}+\dfrac{4}{7}\\ =\left(\dfrac{2}{5}+\dfrac{3}{5}\right)+\left(\dfrac{3}{7}+\dfrac{4}{7}\right)\\ =\dfrac{5}{5}+\dfrac{7}{7}\\ =1+1\\ =2.\)

`2/5+3/7+3/5+4/7`

`=(2/5+3/5)+(3/7+4/7)`

`=5/5+7/7`

`=1+1`

`=2`

` 2/5+3/7+3/5+4/7`

`= ( 2/5+3/5)+(3/7+4/7)`

`=    1 + 1 = 2`

a,19/18    b,27/40

\(\dfrac{2}{5}+\dfrac{4}{5}:4=\dfrac{2}{5}+\dfrac{1}{5}=\dfrac{3}{5}\)

\(\dfrac{3}{8}\times\dfrac{7}{4}-\dfrac{3}{8}=\dfrac{3}{8}\left(\dfrac{7}{4}-1\right)=\dfrac{3}{8}\times\dfrac{3}{4}=\dfrac{9}{32}\)

\(\dfrac{6}{5}-3:\dfrac{15}{4}=\dfrac{6}{5}-\dfrac{4}{5}=\dfrac{2}{5}\)

\(=\dfrac{2}{5}+\dfrac{4}{5}\times\dfrac{1}{4}\\ =\dfrac{2}{5}+\dfrac{1}{5}\\ =\dfrac{3}{5}\\ \\ =\dfrac{3}{8}\times\dfrac{7}{4}-\dfrac{3}{8}\times1\\ =\dfrac{3}{8}\times\left(\dfrac{7}{4}-1\right)\\ =\dfrac{3}{8}\times\dfrac{3}{4}\\ =\dfrac{9}{32}\\\\ =\dfrac{6}{5}-3\times\dfrac{4}{15}\\ =\dfrac{6}{5}-\dfrac{4}{5}\\ =\dfrac{2}{5}\)

\(\dfrac{2}{5}+\dfrac{4}{5}:4=\dfrac{2}{5}+\dfrac{1}{5}=\dfrac{3}{5}\\ \dfrac{3}{8}\times\dfrac{7}{4}-\dfrac{3}{8}=\dfrac{3}{8}\times\dfrac{7}{4}-\dfrac{3}{8}\times1=\dfrac{3}{8}\times\left(\dfrac{7}{4}-1\right)=\dfrac{3}{8}\times\dfrac{3}{4}=\dfrac{9}{32}\\ \dfrac{6}{5}-3:\dfrac{15}{4}=\dfrac{6}{5}-\dfrac{4}{5}=\dfrac{2}{5}\)

Cách giải:

4/7 : 2/5 + 3/7 : 2/5 - 7/9

= ( 4/7 + 3/7 ) : 2/5 - 7/9

= 1 : 2/5 - 7/9

= 5/2 - 7/9

= 31/18.                          **** cho mình nha bạn.

3A=5(3/1*4+3/4*7+...+3/91*94)

3A=5(1-1/4+1/4-1/7+...+1/91-1/94)

3A=5(1-1/94)

3A=5.93/94

A=(5.93/94):3

A=155/94