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.

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

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

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

\(a,\dfrac{3}{4}x-\dfrac{7}{12}=\dfrac{5}{6}-\dfrac{2}{3}\\ \Rightarrow\dfrac{3}{4}x-\dfrac{7}{12}=\dfrac{1}{6}\\ \Rightarrow\dfrac{3}{4}x=\dfrac{1}{6}+\dfrac{7}{12}\\ \Rightarrow\dfrac{3}{4}x=\dfrac{3}{4}\\ \Rightarrow x=\dfrac{3}{4}:\dfrac{3}{4}\\ \Rightarrow x=1\\ b,\dfrac{-5}{x}=\dfrac{20}{28}\\ \Rightarrow\dfrac{-5}{x}=\dfrac{5}{7}\\ \Rightarrow\dfrac{-5}{x}=\dfrac{-5}{-7}\\ \Rightarrow x=-7\\ c,2\dfrac{1}{3}:x=7\\ \Rightarrow\dfrac{7}{3}:x=7\\ \Rightarrow x=\dfrac{7}{3}:7\\ \Rightarrow x=\dfrac{1}{3}\)

\(d,\dfrac{-105}{12}< x< \dfrac{20}{7}\Rightarrow x\in\left\{-8;-7;...;2\right\}\)

a: \(\Leftrightarrow x\cdot\dfrac{3}{4}=\dfrac{3}{4}\)

hay x=1

b: \(\Leftrightarrow x=\dfrac{-28\cdot5}{20}=-7\)

c: \(\Leftrightarrow x=\dfrac{7}{3}:7=\dfrac{1}{3}\)

d: \(\Leftrightarrow-8< x< 3\)

hay \(x\in\left\{-7;-6;-5;-4;-3;-2;-1;0;1;2\right\}\)

a. -13 - (5-x) = -10
-13 - 5 + x = -10
x = -10 + 13 + 5
x = 8
Vậy x = 8
b, -3x - 15 = 2x - 20
-3x - 2x = -20 + 15
-5x = -5
x = -5 : (-5)
x = 1
Vậy x = 1
c, 5 - (x-3)^2 = 20
(x-3)^2 = 5 - 20 
(x-3)^2 = -15 ( vô lí )

Vậy không có giá trị x thỏa mãn
d. | x - 12 | - (-5) = 12
| x - 12 | = 12 + (-5)
| x - 12 | = 7
x - 12  = + - 7
(+) x - 12 = 7           (+) x - 12 = -7
x = 7 + 12                        x     = -7 + 12
X = 19                              x     = 5

Vậy x thuộc { 19 ; 5 }
 

5. ( 12 - x ) = 50

      12 - x   = 50 : 5

      12 - x   = 10

             x   = 12 - 10

             x   = 2

 Vậy x=2

Trả lời câu hỏi xưa

5. ( 12 - x ) - 20 =30

5 . ( 12 - x ) = 30 + 20

5 . ( 12 - x ) = 50

12 - x = 50 : 5

12 - x = 10

x = 12 - 10

x = 2

Vậy x = 2

Bài làm

x + 5 = 20 - ( 12 - 17 )

x + 5 = 20 - ( -5 )

x + 5 = 20 + 5

x + 5 = 25

x       = 25 - 5

x       = 20

Vậy x = 20

# Chúc bạn học tốt #

x+5=20-(12-17)

x+5=20-12+17

x+5=8+17

x+5=25

x=25-5

x=20

Vậy x=20

nhớ tk m nha

a) \(2x+5=20-\left(12-7\right)\)

\(2x+5=20-5\)

\(2x+5=15\)

\(2x=15-5\)

\(2x=10\)

\(x=10:2\)

\(x=5\)

Vậy \(x=5\)

b) \(\left|x\right|=\left|-5\right|\)

\(\Rightarrow\left|x\right|=5\)

\(\Rightarrow x=\pm5\)

Vậy \(x\in\left\{5;-5\right\}\)

a: -12/x=3/5

=>x=-12x5:3=-20

b: (x-1)+(x-2)+(x-3)+...+(x-20)=10

=>20x-210=10

=>20x=220

hay x=11