\(a.\dfrac{12+x}{42}=\dfrac{35}{42}\Leftrightarrow12+x=35\Leftrightarrow x=23\)

\(b.\dfrac{25-x}{40}=\dfrac{3}{8}\Leftrightarrow\dfrac{25-x}{40}=\dfrac{15}{40}\Leftrightarrow25-x=15\Leftrightarrow x=10\)

\(c.\dfrac{13+x}{20}=\dfrac{3}{4}\Leftrightarrow\dfrac{13+x}{20}=\dfrac{15}{20}\Leftrightarrow13+x=15\Leftrightarrow x=2\)

\(d.\dfrac{23-x}{25}=\dfrac{20}{25}\Leftrightarrow23-x=20\Leftrightarrow x=3\)

b) \(23-\frac{x}{25}=\frac{4}{5}\)

\(\frac{x}{25}=23-\frac{4}{5}\)

\(\frac{x}{25}=\frac{111}{5}=\frac{555}{25}\)

=> x = 555

Mình làm cho bạn sao bạn không tick vậy 

a) \(\frac{13+x}{20}=\frac{3}{4}\)

\(\Rightarrow\frac{13+x}{20}=\frac{15}{20}\)

\(\Rightarrow13+x=15\)

\(\Rightarrow\)        \(x=15-13\)

\(\Rightarrow\)\(x=2\)

b)\(\frac{23-x}{25}=\frac{4}{5}\)

\(\Rightarrow\frac{23-x}{25}=\frac{20}{25}\)

\(\Rightarrow23-x=20\)

\(\Rightarrow\)        \(x=23-20\) 

\(\Rightarrow\)        \(x=3\)

a, 13+x/20= 3/4

Suy ra: ( 13+x). 4= 20.3

Suy ra: (13+x).4=60

Suy ra: 13+x=60:4

Suy ra: 13+x=15

Suy ra: x=15-13

Suy ra: x=2

Vậy x=2

a\(\frac{13+x}{20}=\frac{3}{4}\Rightarrow\left(13+x\right)\times4=20\times3\)

\(\left(13+x\right)\times4=60\)

\(13+x=60:4=15\)

\(x=15-13=2\)

Vậy x=2

b,\(\frac{23-x}{25}=\frac{4}{5}=\left(23-x\right)\times5=25\times4\)

\(\left(23-x\right)\times5=100\Rightarrow23-x=100:5=20\)

\(x=23-20=3\)

Vậy x = 3

`(13+x)/20 = 3/4`

`(13+x) xx4=3xx20`

`(13+x)xx4=60`

`13+x=60:4`

`13+x=15`

`x=15-13`

`x=2`

__

`(23-x)/25 =4/5`

`(23-x)xx5=4xx25`

`(23-x)xx5=100`

`23-x=100:5`

`23-x=20`

`x=23-20`

`x=3`

13 +  x/20 = 3/4 

        x/20 = 3/4 - 13 

        x/20 = -49/4

   => x = - 49 x 20 / 4 

        x =  - 245

cái này là theo  ý kiến mk nhá ! 

tk ủng hộ mk nhé !!!!!!

Câu 1 sai đề.

Câu 2:

\(23-\frac{x}{25}=\frac{4}{5}\)

\(\frac{x}{25}=\frac{111}{5}\)

\(x=555\)

Câu 2:

$23-\frac{x}{25}=\frac{4}{5}$

$\frac{x}{25}=\frac{111}{5}$

$x=555$

BÀI 1: Tính

1) -(-10)-(+14)=10-14=-4

2) -(+15)-12=-15-12=-27

3) (-11)-(-13)=-11+13=2

4) -(-13)-(-10)=13+10=23

5) -4-(+7)=-4-7=-11

Bài 2 : tìm x:

a)x+(-5)=-(-7) 

           x=7+(-5)

           x=2

vậy x=2

b)x-8=-(+10)

x-8=10

   x=10+8

  x=18

vậy x=18

c)x-(-12)=14

x+12=14

      x=14-12

      x=2

vậy x=2

d)x-(+3)=-17

x-3=17

   x=17+3

   x=20

vậy x=20

e)x+20=-(-23)

x+20=23

      x=23-20

      x=3

vậy x=3

BÀI 3:tính

a)17-(-9)-(+25)

=17+9-25

=26-25

=1

b)-[-13]-15+(-20)

=13-15-20

=-2-20

=-22

c)-17-(-16)-23

=-17+16-23

=-1-23

=-24

d)-(-25)-(-14)+(-17)

=25+14-17

=39-17

=22

cảm ơn bạn rất nhiều chúc bạn và gia đình mạnh khỏe, vui vẻ bên nhau và cùng đón 1 cái TẾT vui vẻ!

HAPPY NEW YEAR

haha ối giời ơi

BAI 1 ; 

Bài 2: 

a, \(\dfrac{5}{23}\) \(\times\) \(\dfrac{17}{26}\) + \(\dfrac{5}{23}\) \(\times\) \(\dfrac{9}{26}\) 

= \(\dfrac{5}{23}\) \(\times\) ( \(\dfrac{17}{26}\) + \(\dfrac{9}{26}\))

= \(\dfrac{5}{23}\) \(\times\) \(\dfrac{26}{26}\)

= \(\dfrac{5}{23}\) 

b, \(\dfrac{3}{4}\) \(\times\) \(\dfrac{7}{9}\) + \(\dfrac{7}{4}\)  \(\times\) \(\dfrac{3}{9}\)

= \(\dfrac{7}{12}\) + \(\dfrac{7}{12}\)

= \(\dfrac{14}{12}\)

= \(\dfrac{7}{6}\)

1,a) \(\dfrac{17}{20}=\dfrac{51}{60};\dfrac{13}{15}=\dfrac{52}{60}\)

Ta có : 1 + 3 + 5 + ..... + x = 1600

=> \(\frac{\text{[(x - 1) : 2 + 1](x + 1)}}{2}=1600\)

\(\Rightarrow\left(\frac{1}{2}x-\frac{1}{2}+1\right)\left(x+1\right)=3200\)

\(\Rightarrow\left(\frac{1}{2}x+\frac{1}{2}\right)\left(x+1\right)=3200\)

\(\Rightarrow\left(x+1\right)^2.\frac{1}{2}=3200\)

\(\Rightarrow\left(x+1\right)^2=6400\)

\(\Rightarrow x+1=80\)

=> x = 79