Bài toán 3 :

a) 5 – (12 – x) = 129 – (6 + 129)

<=> 5 - 12 + x = 129 - 6 - 129

<=>  x = 129 - 6 - 129 - 5 + 12

<=> x = 1

b) 20 – (x + 3) = - 45 – (17 – 45) 

<=> 20 - x - 3 = -45 - 17 + 45

<=> -x = -45 - 17 + 45 + 3 - 20

<=> -x = -34

<=> x = 34

c) 24 + (17 – x) = (125 – 30) – 125

<=> 24 + 17 - x = 95 - 125

<=> -x = 95 - 125 - 17 - 24

<=> -x = -71

<=> x = 71

d) 3(x – 4) – (8 – x) = 12

<=> 3x - 12 - 8 + x = 12

<=> 3x + x = 12 + 8 + 12

<=> 4x = 32

<=> x = 8

e) x – (13 – x) = - 24 + (-7 + x)

<=> x - 13 + x = -24 - 7 + x

<=> x + x - x = -24 -7 + 13

<=> x = -18

f) (x – 7) + (5 – x) = 12 – (-8 + x)

<=> x - 7 + 5 - x = 12 + 8 - x

<=> x - x + x = 12 + 8 + 7 - 5

<=> x = 22

g) 124 + (13 – 16) = 162 – (x + 162)

<=> 124 + 13 - 16 = 162 - x - 162

<=> x = 162 - 162 - 124 - 13 + 16

<=> x = -121

h) 5(x – 6) – 2(x + 3) = 12

<=> 5x - 30 - 2x - 6 = 12

<=> 5x - 2x = 12 + 6 + 30

<=> 3x = 48

<=> x = 16

Đáp án + Giải thích các bước giải:

Ta có :

`a,5-(12-x)=129-(6+129)`

`→5-12+x=129-129-6`

`→-7+x=-6`

`→x=1`

`b,20-(x+3)=-45-(17-45)`

`→20-x-3=-45+45-17`

`→17-x=-17`

`→x=34`

`c,24+(17-x)=(125-30)-125`

`→24+17-x=125-125-30`

`→41-x=-30`

`→x=71`

`k,3(x-4)-(8-x)=12`

`→3x-12-8+x=12`

`→4x-20=12`

`→4x=32`

`→x=8`

`e,x-(13-x)=-24+(-7+x)`

`→x-13+x=-24-7+x`

`→2x-13=-31+x`

`→2x-x=13-31`

`→x=-18`

`f,(x-7)+(5-x)=12-(-8+x)`

`→x-7+5-x=12+8-x`

`→-2=20-x`

`→x=2+20`

`→x=22`

`g,124+(13-16)=162-(x+162)`

`→124+13-16=162-162-x`

`→121=-x`

`→x=-121`

`h,5(x-6)-2(x+3)=12`

`→5x-30-2x-6=12`

`→3x-36=12`

`→3x=48`

`→x=16`