a) 

(1/3-1/5)x1/4 = 1/3x1/4-1/5x1/4=1/12-1/20=1/30

(1/3-1/5)x1/4 = 2/15x1/4=1/30

b)

2/5 x 3/7 + 2/5 x 4/7= 6/35 + 8/35 = 14/35 = 2/5

2/5 x 3/7 + 2/5 x 4/7= (3/7+4/7) x 2/5 = 1 x 2/5 = 2/5

a: C1: (1/3-1/5)*1/4

=2/15*1/4=2/60=1/30

C2: (1/3-1/5)*1/4

=1/3*1/4-1/5*1/4

=1/12-1/20=1/30

b: C1: 2/5*3/7+2/5*4/7

=6/35+8/35=14/35=2/5

C2: 2/5*3/7+2/5*4/7

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

=2/5*1=2/5

1/2* x+2/3=9/2

1/2 * x = 9/2 - 2/3 

1/2 * x= 23/6

x= 23/6 : 1/2

x= 23/6 x 2= 23/3

___

1/2*x-1/3=2/3

1/2*x = 2/3 + 1/3

1/2 * x= 1

x= 1: 1/2 

x= 2

____

1/4+3/4:x=3

3/4 : x = 3 - 1/4

3/4 : x= 11/4

x= 11/4 : 3/4

x= 11/3

\(\dfrac{1}{2}\)\(\times\)\(x\) + \(\dfrac{2}{3}\) = \(\dfrac{9}{2}\)

\(\dfrac{1}{2}\)\(\times\)\(x\)        = \(\dfrac{9}{2}\) - \(\dfrac{2}{3}\)

\(\dfrac{1}{2}\)\(\times\)\(x\)       = \(\dfrac{23}{6}\)

      \(x\)       = \(\dfrac{23}{6}\):\(\dfrac{1}{2}\)

      \(x\)      = \(\dfrac{23}{3}\) 

\(\dfrac{1}{2}\)\(\times\)\(x\) - \(\dfrac{1}{3}\) = \(\dfrac{2}{3}\) 

\(\dfrac{1}{2}\)\(\times\)\(x\)       = \(\dfrac{2}{3}\) + \(\dfrac{1}{3}\)

\(\dfrac{1}{2}\times\)\(x\)      =  1

     \(x\)       = 1 : \(\dfrac{1}{2}\)

   \(x\)         = 2

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

          \(\dfrac{3}{4}\): \(x\) = 3 - \(\dfrac{1}{4}\) 

          \(\dfrac{3}{4}\):\(x\) = \(\dfrac{11}{4}\)

              \(x\) = \(\dfrac{3}{4}\): \(\dfrac{11}{4}\)

             \(x\) = \(\dfrac{3}{11}\)

` @ L I N H `

1/2* x+2/3=9/2

1/2 * x = 9/2 - 2/3 

1/2 * x= 23/6

x= 23/6 : 1/2

x= 23/6 x 2= 23/3

___

1/2*x-1/3=2/3

1/2*x = 2/3 + 1/3

1/2 * x= 1

x= 1: 1/2 

x= 2

____

1/4+3/4:x=3

3/4 : x = 3 - 1/4

3/4 : x= 11/4

x= 11/4 : 3/4

x= 11/3

ô hô hô

sao mà bị mắng?

hihihihahahaha

\(\frac{1}{2}\times\left(x-\frac{4}{5}\right)+\frac{3}{4}x=\frac{5}{12}\)

\(\Leftrightarrow\frac{1}{2}x-\frac{2}{5}+\frac{3}{4}x=\frac{5}{12}\)

\(\Leftrightarrow\frac{1}{2}x+\frac{3}{4}x=\frac{5}{12}+\frac{2}{5}\)

\(\Leftrightarrow\frac{5}{4}x=\frac{49}{60}\)

\(\Leftrightarrow x=\frac{49}{75}\)

Vậy \(x=\frac{49}{75}\)

bạn ơi sao lại có 2/5 vâỵ

phá ngoặc  \(\frac{1}{2}\times\frac{4}{5}=\frac{4}{10}=\frac{2}{5}\)  nhé bạn

Bài làm:

1) \(\frac{3}{5}\div\frac{2x}{15}=\frac{1}{2}\div\frac{4}{5}\)

\(\Leftrightarrow\frac{9}{2x}=\frac{5}{8}\)

\(\Rightarrow10x=72\)

\(\Leftrightarrow x=\frac{36}{5}\)

2) \(-\frac{4}{2,5}\div\frac{3}{5}=\frac{1}{5}\div x\)

\(\Leftrightarrow\frac{1}{5}\div x=-\frac{8}{3}\)

\(\Rightarrow x=-\frac{3}{40}\)

3) \(0,12\div3=2x\div\frac{3}{5}\)

\(\Leftrightarrow\frac{1}{25}=\frac{10}{3}x\)

\(\Rightarrow x=\frac{3}{250}\)