`5/9 xx a/b -11/21 =4/21`

`=> 5/9 xx a/b = 4/21 + 11/21`

`=> 5/9 xx a/b=15/21`

`=> a/b= 15/21 : 5/9`

`=>a/b= 15/21 xx 9/5`

`=> a/b=135/105`

`=>a/b=9/7`

Vậy `a/b=9/7`

\(\dfrac{5}{9}\times\dfrac{a}{b}-\dfrac{11}{21}=\dfrac{4}{21}\)

\(\Leftrightarrow\dfrac{5}{9}\times\dfrac{a}{b}=\dfrac{15}{21}\)

\(\Leftrightarrow\dfrac{a}{b}\times\dfrac{5}{9}=\dfrac{15}{21}\)

\(\Leftrightarrow\dfrac{a}{b}=\dfrac{15}{21}:\dfrac{5}{9}\)

\(\Leftrightarrow\dfrac{a}{b}=\dfrac{135}{105}=\dfrac{9}{7}\)

vaayjj : \(\dfrac{a}{b}=\dfrac{9}{7}\)

\(\dfrac{5}{9}\times\dfrac{a}{b}-\dfrac{11}{21}=\dfrac{4}{21}\\ \Rightarrow\dfrac{5}{9}\times\dfrac{a}{b}=\dfrac{4}{21}+\dfrac{11}{21}\\ \Rightarrow\dfrac{5}{9}\times\dfrac{a}{b}=\dfrac{5}{7}\\ \Rightarrow\dfrac{a}{b}=\dfrac{5}{7}\div\dfrac{5}{9}\\ \Rightarrow\dfrac{a}{b}=\dfrac{5}{7}\times\dfrac{9}{5}\\ \Rightarrow\dfrac{a}{b}=\dfrac{9}{7}\)

Vậy \(\dfrac{a}{b}=\dfrac{9}{7}\)

Bài 1:

a) \(\frac{16}{15}.\frac{\left(-5\right)}{14}.\frac{54}{24}.\frac{56}{21}\)

\(=\frac{4.2.2}{5.3}.\frac{\left(-5\right)}{2.7}.\frac{3.3}{4}.\frac{8}{3}\)

\(=\frac{4.2.2.\left(-5\right).3.3.8}{5.3.2.7.4.3}\)

\(=\frac{-16}{7}\)

b) \(\frac{7}{3}.\frac{\left(-5\right)}{2}.\frac{15}{21}.\frac{4}{\left(-5\right)}\)

\(=\frac{7}{3}.\frac{\left(-5\right)}{2}.\frac{5}{7}.\frac{2.2}{\left(-5\right)}\)

\(=\frac{7.\left(-5\right).5.2.2}{3.2.7.\left(-5\right)}\)

\(=\frac{10}{3}\)

Bài 2:

a) \(\frac{21}{24}.\frac{11}{9}.\frac{5}{7}=\frac{7}{8}.\frac{11}{9}.\frac{5}{7}=\frac{11.5}{8.9}=\frac{55}{72}\)

b) \(\frac{5}{23}.\frac{17}{26}+\frac{5}{23}.\frac{9}{26}\)

\(=\frac{5}{23}.\left(\frac{17}{26}+\frac{9}{26}\right)=\frac{5}{23}.1=\frac{5}{23}\)

c) \(\left(\frac{3}{29}-\frac{1}{5}\right).\frac{29}{3}=\frac{3}{29}.\frac{29}{3}-\frac{1}{5}.\frac{29}{3}\)

\(=1-1\frac{14}{15}=\frac{14}{15}\)

Bài 3:

a) x/5 = 2/5

=> x =2

b) -4/x = 20/14 = 10/7

=> -4/x = 10/7

=> x.10 = (-4).7

x.10 = - 28

x= -28 :10

x= -2,8

c) 4/7 = 12/x = 12/ 21

=> 12/x = 12/21

=> x = 21

d) 3/7 = x / 21 = 9/21

=> x/21 = 9/21

=> x= 9

a, = \(\frac{11}{21}\)x \(\left(\frac{7}{4}+\frac{5}{4}\right)\)

   = \(\frac{11}{21}\)x 3

  = \(\frac{33}{21}\)

b) = \(\left(\frac{23}{14}-\frac{9}{14}\right)\)x \(\frac{6}{13}\)

    = 1 x \(\frac{6}{13}\)

    = \(\frac{6}{13}\)

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Lời giải:
a.

$x=\frac{7}{25}+\frac{-1}{5}=\frac{7}{25}+\frac{-5}{25}=\frac{7-5}{25}=\frac{2}{25}$

b.

$x=\frac{5}{11}+\frac{4}{-9}=\frac{5}{11}-\frac{4}{9}=\frac{45}{99}-\frac{44}{99}=\frac{1}{99}$

c.

$\frac{x}{-1}=\frac{-1}{3}-\frac{5}{9}=\frac{-3}{9}-\frac{5}{9}=\frac{-8}{9}$

$x=(-1).\frac{-8}{9}=\frac{8}{9}$

mn. giúp em nha 

\(a.\dfrac{6}{5}=\dfrac{18}{x}\Rightarrow x=\dfrac{18\cdot5}{6}=15\\ \text{Vậy}\text{ }x=15.\)

\(b.\dfrac{3}{4}=\dfrac{-21}{x}\Rightarrow x=\dfrac{-21\cdot4}{3}=28\\ \text{ }\text{ }\text{ }\text{ }\text{Vậy }x=28.\)

\(c.\dfrac{x}{4}=\dfrac{21}{28}\Rightarrow x=\dfrac{21\cdot4}{28}=3\\ \text{Vậy }x=3.\)

\(d.\dfrac{-8}{2x}=\dfrac{3}{-9}\Rightarrow x=\dfrac{-8\cdot\left(-9\right)}{3}:2=12\\ \text{Vậy }x=12.\)

\(e.\dfrac{-4}{11}=\dfrac{x}{22}=\dfrac{40}{z}\\ \Rightarrow x=\dfrac{-4\cdot22}{11}=-8\\ \Rightarrow z=\dfrac{22\cdot40}{-8}=-110\\ \text{Vậy }x=-8;z=-110.\)

\(f.\dfrac{-3}{4}=\dfrac{x}{20}=\dfrac{21}{y}\\ \Rightarrow x=\dfrac{-3\cdot20}{4}=-15\\ \Rightarrow y=\dfrac{21\cdot20}{-15}=-28\\ \text{Vậy }x=-15;y=-28.\)

\(g.\dfrac{-4}{8}=\dfrac{x}{-10}=\dfrac{-7}{y}=\dfrac{z}{-24}\\ \Rightarrow x=\dfrac{-4\cdot\left(-10\right)}{8}=5\\ \Rightarrow y=\dfrac{-7\cdot\left(-10\right)}{5}=14\\ \Rightarrow z=\dfrac{-7\cdot\left(-24\right)}{14}=12\\ \text{Vậy }x=5;y=14;z=12.\)

\(h.\dfrac{x}{4}=\dfrac{9}{x}\\ \Rightarrow x\cdot x=9\cdot4\\ \Rightarrow x\cdot x=36\\ \Rightarrow x\cdot x=6\cdot6\\ \text{Vậy }\text{cả hai }x=6.\)

a  \(\dfrac{5}{12}\times\dfrac{4}{9}\) + \(\dfrac{4}{9}\times\dfrac{7}{12}\)

= \(\dfrac{4}{9}\times\left(\dfrac{5}{12}+\dfrac{7}{12}\right)\)

= \(\dfrac{4}{9}\times\dfrac{12}{12}\)

= \(\dfrac{4}{9}\)

b(\(\dfrac{13}{21}\times\dfrac{7}{8}-\dfrac{12}{21}\times\dfrac{7}{8}\)) x 12 

(\(\dfrac{7}{8}\times\left(\dfrac{13}{21}-\dfrac{12}{21}\right)\))x 12

\(\dfrac{7}{8}\times\dfrac{1}{21}\) x 12

\(\dfrac{139}{14}\)

\(\dfrac{7}{11}+\dfrac{3}{4}+\dfrac{4}{11}+\dfrac{8}{17}+\dfrac{1}{4}+\dfrac{18}{34}\)

=\(\left(\dfrac{7}{11}+\dfrac{4}{11}\right)+\left(\dfrac{3}{4}+\dfrac{1}{4}\right)\) + \(\dfrac{18}{34}\) 

=1 + 1 + \(\dfrac{18}{34}\)

=\(\dfrac{34}{17}\)

2)a 3,71 + (632 - x) = 415 

                  632 - x= 415 - 3,71 

                  632 -x = 411,59

                          x = 632 - 411,59 

                          x = 220,41

b, 140 : (5x \(x\) - 15) = 4 

              5x \(x\) - 15 = 140 : 4

             5x \(x\) - 15 = 35 

              5x \(x\) = 35 + 15 

             5x \(x\) = 50 

                  \(x\) = 50 : 5 

                 \(x\) = 10

a) => y+42+2y= -12-14+2y

y+2y-2y = -12-14-42

y= -68

b) => 15+y-5-5y= -12-5y

y-5y+5y= -12-15+5

y = -22

c) => 2y+5-8y+21= -3-5y-2

2y-8y+5y= -3-2-5-21

-y= -31=>y=31

d)=> -13+3y+23= -120+y

3y-y= -120+13-23

2y= -130=>y= -65

e) => -21+32+5y= 16+4y

5y-4y= 16+21-32

y= 5

bài 1

a)y-(-42-2y) = (-12) - 14 +2y

y +42 + 2y = -12 -14 +2y

3y + 42 = -26 +2y

y = -68

b)15-(-y+5)-5y=-(12+5y+2)

15+y-5-5y=-12-5y-2

10-4y=-14-5y

-4y+5y=-14-10=-24

c)2y-(-5+8y-21)=-3-(5y+2)

2y+5-8y+21=-3y-5y-2

-6y+26=-8y-2

-6y+8y=-2-26

2y=-28

y=-28/2=-14

\(\frac{5}{9}\) . \(\frac{a}{b}\) - \(\frac{11}{21}\) = \(\frac{4}{21}\)  

\(\frac{5}{9}\) . \(\frac{a}{b}\)               = \(\frac{4}{21}\) + \(\frac{11}{21}\)

\(\frac{5}{9}\) . \(\frac{a}{b}\)               = \(\frac{5}{7}\) 

            \(\frac{a}{b}\)               = \(\frac{5}{7}\) : \(\frac{5}{9}\) 

              \(\frac{a}{b}\)             = \(\frac{9}{7}\) 

Vậy \(\frac{a}{b}\) = \(\frac{9}{7}\) 

5/9 x a/b-11/21=4/21

<=> 5/9 x a/b=4/21+11/21

<=> 5/9 x a/b = 21/21=1

<=> a/b = 1 : 5/9

<=> a/b=9/5

Vậy a/b=9/5

Ta có : 

\(\frac{5}{9}\times\frac{a}{b}-\frac{11}{21}=\frac{4}{21}\)

\(\Rightarrow\frac{5}{9}\times\frac{a}{b}=\frac{4}{21}+\frac{11}{21}\)

\(\Rightarrow\frac{5}{9}\times\frac{a}{b}=\frac{15}{21}\)

\(\Rightarrow\frac{a}{b}=\frac{15}{21}:\frac{5}{9}\)

\(\Rightarrow\frac{a}{b}=\frac{15}{21}\times\frac{9}{5}\)

\(\Rightarrow\frac{a}{b}=\frac{9}{7}\)

Vậy \(\frac{a}{b}=\frac{9}{7}\)

~ Ủng hộ nhé