9. Tìm x biết 

a x- ( 12- 15)= -8

<=> x - 12 + 15 = -8

<=> x + 3 = -8

<=> x = -11

Vậy x = -11

b. (x- 29 )- ( 17- 38)= -9

<=> x - 29 - 17 + 38 = -9

<=> x - 8 = -9

<=> x = -1

Vậy x = -1

a) x-12+15=-8

=>x=-8-15+12

=>x=-11

Vậy x=-11

b)x-29+21=-9

=>x=-9-21+29

=>x=-1

Vậy x=-1

a) Ta có: x-(12-15)=-8

\(\Leftrightarrow x-12+15=-8\)

\(\Leftrightarrow x+3=-8\)

hay x=-8-3=-11

Vậy: x=-11

b) Ta có: \(\left(x-29\right)-\left(17-38\right)=-9\)

\(\Leftrightarrow x-29-17+38+9=0\)

\(\Leftrightarrow x+1=0\)

hay x=-1

Vậy: x=-1

Bài 3:

a: A=-5-7-8=-20

b: =-3+7+2*(-8)=-16+4=-12

c: =2-3*15-8=-55

Bài 4:

a: x-y+2=-12-25+2=-37+2=-35

b: x+y+z-y-3=x+z-3=-12-15-3=-30

c: =20-12-(25-15)+12=20-10=10

d: =25-(-12+25+15)+3

=25+12-25-15-3

=-3-3=-6 

`x/8 = 3/4 +(-5/8)`

`=>x/8 = 6/8 +(-5/8)`

`=>x/8 = 1/8`

`=>x=1`

`-----`

`x/12 =3/4 +(-2/3)`

`=>x/12 = 9/12 + (-8/12)`

`=> x/12=1/12`

`=>x=1`

`----`

`1+11/13=24/x`

`=> 13/13 +11/13=24/x`

`=> 24/13 =24/x`

`=>x=13`

`----`

`x/6 -3/4=1/12`

`=>x/6 = 1/12 +3/4`

`=>x/6 = 1/12 + 9/12`

`=>x/6 = 10/12`

`=>x/6= 5/6`

`=>x=5`

\(a)\)

\(\dfrac{x}{8}=\dfrac{3}{4}+\dfrac{-5}{8}\)

\(\Rightarrow\dfrac{x}{8}=\dfrac{1}{8}\)

\(\Rightarrow x.8=1.8\)

\(\Rightarrow x.8=8\)

\(\Rightarrow x=8:8\)

\(\Rightarrow x=1\)

\(b)\)

\(\dfrac{x}{12}=\dfrac{3}{4}+\dfrac{-2}{3}\)

\(\Rightarrow\dfrac{x}{12}=\dfrac{1}{12}\)

\(\Rightarrow x.12=1.12\)

\(\Rightarrow x.12=12\)

\(\Rightarrow x=12:12\)

\(\Rightarrow x=1\)

\(c)\)

\(1+\dfrac{11}{13}=\dfrac{24}{x}\)

\(\Rightarrow\dfrac{24}{13}=\dfrac{24}{x}\)

\(\Rightarrow x.24=24.13\)

\(\Rightarrow x.24=312\)

\(\Rightarrow x=312:24\)

\(\Rightarrow x=13\)

\(d)\)

\(\dfrac{x}{6}-\dfrac{3}{4}=\dfrac{1}{12}\)

\(\Rightarrow\dfrac{x}{6}=\dfrac{1}{12}+\dfrac{3}{4}\)

\(\Rightarrow\dfrac{x}{6}=\dfrac{5}{6}\)

\(\Rightarrow x.6=5.6\)

\(\Rightarrow x.6=30\)

\(\Rightarrow x=30:6\)

\(\Rightarrow x=5\)

\(-12-13+x=-15+17\)

\(-25+x=2\)

\(x=2-\left(-25\right)=2+25=27\)

a) 3+x=-8

x=-8+3

x=-5

b: =>25-x=29

hay x=-4

a) 3 + x = -8                b) (25 - x) - 12 = 27                       c) 2^x + 15 = 31

          x = -8 - 3                25 - x          = 27 + 12                   2^x         = 31 - 15 

          x = -11                   25 - x          = 39                            2^x         = 16

Vậy x = -11                             x          = 25 - 39                     2^x         = 2⁴

                                               x           = - 14                      => x = 4

                                          Vậy x = -14                               Vậy x = 4

\(\frac{x+22}{11}+\frac{x+23}{12}=\frac{x+24}{13}+\frac{x+25}{14}\)

\(\Leftrightarrow\left(\frac{x+22}{11}+1\right)+\left(\frac{x+23}{12}+1\right)=\left(\frac{x+24}{13}+1\right)+\left(\frac{x+25}{14}+1\right)\)

\(\Leftrightarrow\frac{x+33}{11}+\frac{x+35}{12}=\frac{x+37}{13}+\frac{x+39}{14}\)

\(\Leftrightarrow\frac{x+33}{11}+\frac{x+35}{12}-\frac{x+37}{13}-\frac{x+39}{14}=0\)

\(\Leftrightarrow\frac{2184\cdot\left(x+33\right)+2002\cdot\left(x+35\right)-1848\cdot\left(x+37\right)-1716\cdot\left(x+39\right)}{24024}=0\)

\(\Leftrightarrow2184x+72072+2002x+70070-1848x-68376-1716x-66924=0\)

\(\Leftrightarrow622x+6842=0\)

\(\Leftrightarrow x=-11\)

\(x:\dfrac{13}{16}=\dfrac{5}{-8}\\ x=\dfrac{5}{-8}.\dfrac{13}{16}=\dfrac{65}{-128}\)

\(x.\dfrac{-1}{2}=\dfrac{-4}{5}\\ x=\dfrac{-4}{5}:\dfrac{-1}{2}=\dfrac{8}{5}\)

Câu 14: 

a. \(\text{3 + x = - 8}.\) \(\Leftrightarrow x=-8-3=-11.\)

b. \(\text{(35 + x) - 12 = 27}.\) 

\(\Leftrightarrow35+x=27+12.\)

\(\Leftrightarrow35+x=39.\Leftrightarrow x=39-35=14.\)

c. \(2^x+15=31.\) 

\(\Leftrightarrow2^x=16.\)

\(\Leftrightarrow2^x=2^4.\)

\(\Leftrightarrow x=4.\)