a: =>x=7-20=-13

b: =>x=-18+12=-6

c: =>x=9 hoặc x=-6

d: =>x=0 hoặc x=4

e: =>6-x=13-3+14=24

=>x=-18

Câu g và h đề thiếu rồi bạn

`a)`

`x-15 = -21`

`<=> x = -21 + 15`

`<=> x = -6`

`b)`

`42 - x = -7`

`<=> x = 42 - (-7)`

`<=> x = 42 + 7`

`<=> x = 49`

`c)`

`12 - (30-x) = -23`

`30-x = 12 - (-23)`

`30-x = 35`

`x = 30-35`

`x = -5`

`d)`

`31 - (17+x)=18`

`<=> 17+x = 31-18`

`<=> 17+x =13`

`<=> x = 13-17`

`<=> x = -4`

a) \(x-15=-21\)

\(\Rightarrow x=-21+15=-6\)

b) \(42-x=-7\)

\(\Rightarrow x=42+7=49\)

c) \(12-\left(30-x\right)=-23\)

\(\Rightarrow30-x=12+23=35\)

\(\Rightarrow x=30+35=65\)

d) \(31-\left(17+x\right)=18\)

\(\Rightarrow17+x=31-18=13\)

\(\Rightarrow x=13-17=-4\)

a) $x-15=-21$

$\Rightarrow x=-21+15=-6$

b) $42-x=-7$

$x=42+7=49$

c) $12-\left(30-x\right)=-23$

$30-x=12+23=35$

$x=30+35=65$

d) $31-\left(17+x\right)=18$

$17+x=31-18=13$

$x=13-17=-4$

a)\(x-15=3\Leftrightarrow x=3+15\Leftrightarrow x=18\)

b) \(2x+12=36\Leftrightarrow2x=24\Leftrightarrow x=12\)

c)\(\left(x+21\right):8+12=21\)

\(\Leftrightarrow\)\(\frac{x+21}{8}=9\)

\(\Leftrightarrow x+21=72\)

\(\Leftrightarrow x=51\)

d)\(\left(3x+18\right)\left(x-9\right)=0\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}3x-18=0\\x-9=0\end{array}\right.\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}x=6\\x=9\end{array}\right.\)

i) => /x+3/ = 3

=> x+3= 3

     x+3= -3

=> x= 3-3= 0

     x= -3-3 = -6

tick nha

nhiều qua ak, đăng từng câu dễ nhìn hơn đó, làm vậy nhìn thấy là nản ko muốn làm ak

cần giải chi tiết không bạn?

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) 2 . x + 12 = 36

2 . x = 36 - 12

2 . x = 24

x = 24 : 2

x = 12

Vậy x = 12

b) (x + 21)  : 8 + 12 = 21

( x + 21 ) : 8 = 21 - 12

( x + 21 ) : 8 = 9

x + 21 = 9 . 8

x + 21 = 72

x = 72 - 21

x = 51

Vậy x = 51

c) (3 . x - 18) . (x - 9) = 0

\(\Rightarrow\begin{cases}3.x-18=0\\x-9=0\end{cases}\)

\(\Rightarrow\begin{cases}3.x=18\\x=0+9\end{cases}\)

\(\Rightarrow\begin{cases}x=18:3=6\\x=9\end{cases}\)

Vậy \(x\in\left\{6;9\right\}\)

a)

\(2x+12=36\)

\(\Rightarrow2\left(x+6\right)=36\)

\(\Rightarrow x+6=18\)

=> x = 12

Vậy x = 12

b)

\(\left(x+21\right):8+12=21\)

\(\Rightarrow\left(x+21\right):8=9\)

\(\Rightarrow x+21=72\)

=> x = 51

Vậy x = 51

c)

\(\left(3x-18\right)\left(x-9\right)=0\)

\(\Rightarrow3\left(x-6\right)\left(x-9\right)=0\)

\(\Rightarrow\left(x-6\right)\left(x-9\right)=0\)

\(\Rightarrow\left[\begin{array}{nghiempt}x-6=0\\x-9=0\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}x=6\\x=9\end{array}\right.\)

Vậy x = 6 ; x = 9

a) \(2x+12=36\Leftrightarrow2x=24x\Leftrightarrow x=12\)

b) \(x+21:8+12=21\)

\(\Leftrightarrow\frac{x+21}{8}=9\)

\(\Leftrightarrow x+21=72\)

\(\Leftrightarrow x=51\)

c) \(\left(3.x-18\right).\left(x-9\right)=0\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}3x-18=0\\x-9=0\end{array}\right.\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}x=6\\x=9\end{array}\right.\)

Chúc bạn học tốt 

\(x=\dfrac{1}{2}+\left(\dfrac{7}{12}:\dfrac{21}{18}\right)=\dfrac{1}{2}+\left(\dfrac{7}{12}\times\dfrac{18}{21}\right)=\dfrac{1}{2}+\dfrac{1}{2}=\dfrac{2}{2}=1\)

X-7/12 : 21/18 = 1/2

X-7/12             =1/2 x21/18

X-7/12             =7/12

X                      = 7/12 + 7/12

X                      = 7/6

X-7/12 : 21/18 = 1/2

X-7/12 = 1/2 x 21/18

X-7/12 = 7/12

X=7/12+7/12

X=14/12

X=7/6

Đáp số:

a) x=45

b) x=31

c) x=2

d) x=94