Bài 1:

a. $(-20)+x=-30$

$x-20=-30$

$x=-30+20=-(30-20)=-10$

b.

$(-10)-x=-20$

$x=(-10)-(-20)=-10+20=20-10=10$

c. Đề sai. Bạn xem lại.

d.

$x+(-3)=-7$

$x=-7-(-3)=-7+3=-(7-3)=-4$

e.

$x-(-5)=-9$

$x=(-9)+(-5)=-14$

f.

$x(-11)=12$

$x=\frac{12}{-11}=\frac{-12}{11}$

h.

$2x-10=20$

$2x=20+10=30$

$x=30:2=15$

l.

$4x-8=-8$
$4x=-8+8=0$

$x=0:4=0$

k.

$-12-(-2)x=-8$

$(-2)x=-12-(-8)=-12+8=-(12-8)=-4$

$x=(-4):(-2)=2$

Bài 2:

a. $-20-(10-x)=-3$

$10-x=-20-(-3)=-20+3=-(20-3)=-17$

$x=10-(-17)=10+17=27$

b.

$14+(14-x)=-2$
$14-x=-2-14=-16$

$x=14-(-16)=14+16=30$

c.

$-15-(x-3)=-7$

$x-3=-15-(-7)=-15+7=-8$

x=-8+3=-5$

d.

$(x+4)+(-20)=-8$

$x+4=-8-(-20)=-8+20=12$
$x=12-4=8$

e.

$-2x-2=-4$

$-2x=-4+2=-2$

$x=(-2):(-2)=1$

f.

$-2x+4=-4$

$-2x=-4-4=-8$

$x=(-8):(-2)=4$

l.

$-12-(-2)x=-2-4=-6$

$(-2)x=-12-(-6)=-12+6=-6$

$x=(-6):(-2)=3$

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Phân biệt x và dấu nhân kiểu gì?

a)x+10=30

=>x=20

b)x+10-5=20+20

x+10-5=40

x+10=45

=>x=35

x + 10 = 50 - 20

x + 10 = 30

x = 20 

x + 10 - 5 = 20 + 20

x + 5 = 40

x = 35 

\(x+10=50-20\)

\(x+10=30\)

\(x=30-10\)

\(x=20\)

\(x+10-5=20+20\)

\(x+5=40\)

\(x=40-5\)

\(x=35\)

10 + 20 = 50 - x 

30 = 50 - x 

x = 50 - 30

x = 20 

10 + 20 = 50 - x

30 = 50 - x

x = 20

10 + 90 = 10 + x

100 = 10 + x

x = 90

\(10+20=50-x\)

\(30=50-x\)

   \(x=50-30\)

   \(x=20\)

\(10+90=10+x\)

\(100=10+x\)

     \(x=100-10\)

     \(x=90\)

10 + 10 = 30 - x

20 = 30 - x

x = 30 - 20

x = 10

20 - 10 = 100 - x 

10 = 100 - x

x = 100 - 10

x = 90

Tìm x :

10 + 10 = 30 - x

20 - 10 = 100 - x

10 + 10 = 30 - x

20 = 30 - x

30 - x = 20

x = 30 - 20 = 10

20 - 10 = 100 - x

10= 100 - x 

100 - x = 10

x = 100 - 10 = 90

10+10=30-x

=>30-x=10+10

=>30-x=20

=>x=30-20

=>x=10

20-10=100-x

=>100-x=20-10

=>100-x=10

=>x=100-10

=>x=90

a) (125 - x) + 25 = 98

125 - x = 98 - 25

125 - x = 73

x = 125 - 73

x = 52

b) 100 : (x + 20) = 68 : 34

100 : (x + 20) = 2

x + 20 = 100 : 2

x + 20 = 50

x = 50 - 20

x = 30

c) (3x + 12) . 5 = 25 . 4 - 10

(3x + 12) . 5 = 100 - 10

(3x + 12) . 5 = 90

3x + 12 = 90 : 5

3x + 12 = 18

3x = 18 - 12

3x = 6

x = 6 : 2

x = 3

d) 210 : (2x - 3) - 20 = 10

210 : (2x - 3) = 10 + 20

210 : (2x - 3) = 30

2x - 3 = 210 : 30

2x - 3 = 70

2x = 70 + 3

2x = 73

x = 73/2

a, ( 125 - x ) + 25 = 98

⇒ 125 - x = 73

⇒ x = 52.

Vậy..

b, 100 : ( x + 20 ) = 68 : 34

⇒ 100 : ( x + 20 ) = 2

⇒ x + 20 = 50

⇒ x = 30

Vậy...

c, ( 3 . x + 12 ) . 5 = 25 . 4 - 10

⇒ ( 3 . x + 12 ) . 5 = 90

⇒ 3 .x + 12 = 18

⇒ 3x = 6

⇒ x = 2.

Vậy...

d, 210 : ( 2 .x - 3 ) - 20 = 10

⇒ ( 2 .x - 3 ) - 20 = 21

⇒ 2x - 3 = 41

⇒ 2x = 44

⇒ x = 22.

Vậy..

Cho mk chỉnh lại câu cuối:

 d, 210 : ( 2 .x - 3 ) - 20 = 10

⇒ 210 : ( 2 .x - 3 ) = 30

⇒ 2 . x - 3 = 7

⇒ 2x = 10

⇒ x = 5.

Vậy..

x+20+20=200+200-10

x+20+20=400-10

x+20+20=390

x+40      =390

            x=390-40

            x=350

chuc bn hoc gioi!

x + 20 + 20 = 200 + 200 - 10

x + 20 + 20 = 390

x + 40 = 390

        x = 390 - 40

        x = 350

x + 20 + 20 = 200 + 200 - 10

x + 20 + 20 = 390

x + 40 = 390

        x = 390 - 40

        x = 350

k mik 

a. x × 3 = 27

x = 27 : 3

x = 9

b. 4 × x = 20

x = 20 : 4

x = 5

c. 10 + x : 2 = 20

x : 2 = 20 – 10

x : 2 = 10

x = 10 × 2

x = 20

d. x × 3 = 27 + 3

x × 3 = 30

x = 30 : 3

x = 10

e. 27 : x = 789 – 780

27 : x = 9

x = 27 : 9

x = 3

x^2=16

Suy ra x^2=4^2

Vậy x^2=4^2

[1/(x+2)-1/(x+5)]+[1/(x+5)-1/(x+10)]+[1/(x+10)-1/(x+17)]=x/15.[1/(x+2)-1/(x+17)]
1/(x+2)-1/(x+17)=x/15.[1/(x+2)-1/(x+17)]
1=x/15
x=15

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