Bài 1: 

a: x/-2=-18/x

=>x²=36

=>x=6 hoặc x=-6

b: x/2+x/5=17/10

=>7/10x=17/10

hay x=17/7

17/18

13/6

30/28

16/5

5/9 + 7/18 = 10/18 + 7/18 = 17/18
3 - 5/6= 18/6 - 5/6 = 13/6
5/4 x 6/7 = 15/14
2 : 5/8 = 2/1 x 8/5 = 16/5

\(\dfrac{5}{9}+\dfrac{7}{18}=\dfrac{10}{18}+\dfrac{7}{18}=\dfrac{17}{18}\)

\(\dfrac{18}{6}-\dfrac{5}{6}=\dfrac{13}{6}\)

\(\dfrac{5}{4}\times\dfrac{6}{7}=\dfrac{30}{28}=\dfrac{15}{14}\)

\(2\times\dfrac{8}{5}=\dfrac{16}{5}\)

`x xx 6/7=5/14`

`=>x=5/14:6/7`

`=>x=5/14xx7/6`

`=>x=35/84`

`=>x=5/12`

Vậy `x=5/12`

__

`x:2/3=4/9`

`=>x=4/9xx2/3`

`=>x=8/27`

Vậy `x=8/27`

__

`x-1/4=3/2`

`=>x=3/2+1/4`

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

`=>x=7/4`

Vậy `x=7/4`

__

`x+4/5=8/9`

`=>x=8/9-4/5`

`=>x=40/45-36/45`

`=>x=4/45`

Vậy `x=4/45`

\(x\cdot\dfrac{6}{7}=\dfrac{5}{14}\)

\(x\)         \(=\dfrac{5}{14}:\dfrac{6}{7}\)

\(x\)           \(=\dfrac{5}{12}\)

\(x:\dfrac{2}{3}=\dfrac{4}{9}\)

\(x\)        \(=\dfrac{4}{9}\cdot\dfrac{2}{3}\)

\(x\)          \(=\dfrac{8}{27}\)

\(x-\dfrac{1}{4}=\dfrac{3}{2}\)

\(x\)          \(=\dfrac{3}{2}+\dfrac{1}{4}\)

\(x\)            \(=\dfrac{7}{4}\)

\(x+\dfrac{4}{5}=\dfrac{8}{9}\)

\(x\)          \(=\dfrac{8}{9}-\dfrac{4}{5}\)

\(x\)            \(=\dfrac{4}{45}\)

x=-216/917

x=-216/917

các bạn giúp mình nhé

Bài 3: 

a: =>x/56=3/7

hay x=21

b: =>3/x=6/14

hay x=7

c: =>35/x=7/15

hay x=75

d: =>2x=-10/9

hay x=-10/18=-5/9

a)\(2.x-138=8.9\)

\(2.x-138=72\)

\(2.x=72+138\)

\(2.x=210\)

\(x=210\div2\)

\(x=105\)

b)\(\left\{\left(6x-72\right)\div2-84\right\}.28=5628\)

\(\left(6x-72\right)\div2-84=201\)

\(\left(6x-72\right)\div2=285\)

\(6x-72=570\)

\(6x=642\)

\(x=107\)

c)\(420+65.4=\left(x+175\right)\div5+30\)

\(420+260=\left(x+175\right)\div5+30\)

\(680=\left(x+175\right)\div5+30\)

\(650=\left(x+175\right)\div5\)

\(3250=x+175\)

\(3075=x\)

a) 2 * x - 138 = 72

    2 * x          = 72 + 138

    x                = 210 \ 2

    x                = 105

Mk thay x=y nha!!!

a)2xy-138=8x9

2xy-138=72

2xy=72+138

2xy=210

y=210:2

y=105

b){(6xy-72):2-84)}x28=5628

{(6xy-72):2-84)}x28=5628

{(6xy-72):2-84=201

{(6xy-72):2=285

(6xy-72)=570

6y=570+72

6y=642

=>y=107

c)420+65x4=(y+175):5+30

680=(y+175):5+30

650=(y+175):5

3250=y+175

y=3250-175

y=3075

Bài 1 :

\(A=1+2+3+4+5+6+7+8+9\)

   \(=\left(1+9\right)+\left(2+8\right)+\left(3+7\right)+\left(4+6\right)+5\)

   \(=10+10+10+10+5\)      

   \(=45\)

Bài 2 bạn tự làm nha

Bài 2 : Tìm x :

a) ( x - 8 )⁵ = ( x - 8 )⁹

=> x - 8 thuộc { 0 ; 1 } vì :

Nếu x - 8 = 0 thì ( x - 8 )⁵ = 0⁵ = 0

Nếu x - 8 = 0 thì ( x - 8 )⁹ = 0⁹ = 0

Vậy x - 8 = 0 là thỏa mãn điều kiện.

Nếu x - 8 = 1 thì ( x - 8 )⁵ = 1⁵ = 1

Nếu x - 8 = 1 thì ( x - 8 )⁹ = 1⁹ = 1

Vậy x - 8 = 1 là thỏa mãn điều kiện.

=> x - 8 thuộc { 0 ; 1 }

=> x thuộc { 8 ; 9 }

Bài 2 : Tìm x :

b) ( x + 1 ) + ( x + 2 ) +.....+ ( x + 100 ) = 5750

    100x + ( 1 + 2 + 3 + 4 + .....+ 100 )   = 5750

    100x +                  5050                    = 5750

    100x                                                = 5750 - 5050

    100x                                                 =      700

         x                                                  = 700 : 100

         x                                                   =      7