Bài 2: Tìm x, biết: a) 2x – 5 = 7 b) (x + 75) – 30 = 80 c) 70 – (35 + x) = 25 d) 2x – 138 = 2 3 . 3 2 e) 25 + 3(x – 8) = 106 f) 135 – 5(x + 4) = 35 g) 4(x – 3) = 7 2 –1 10 h) (3x – 1) 3 = 8 i) 15 – x = – 45 k) |x – 3 | = 9 l) |x + 4| –2 = 5 m) x – [42 +(–28)] = – 8
a) 2x-5=7
2x=7+5
2x=12
x=12:2=6
Vậy x=6
b) (x+75)-30=80
(x+75)=80+30
(x+75)=110
x=110-75=35
Vậy x=35
c) 70-(35+x)=25
(35+x)=70-25
(35+x)=45
x=45-35=10
Vậy x=10
d) 2x-138=23.32
2x-138=136
2x=136+138
2x=274
x=274:2=137
Vậy x=137
e) 25+3( x-8) =106
25+(x-8)=106-3
25+(x-8)=103
(x-8)=103-25
(x-8)=78
x=78+8=86
Vậy x=86
f) 135-5(x+4)=35
135-(x+4)=35:5
135-(x+4)=7
(x+4)=135-7
(x+4)=128
x=128-4=124
Vậy x=124
g) 4(x-3) =72-110
4(x-3)=-38
(x-3)=(-38):4
(x-3)=-38/4
x=(-38/4)+3=-13/2
Vậy x=-13/2
h) (3x-1).3=8
(3x-1)=8:3
(3x-1)=8/3
3x-1=8/3
3x=8/3+1
3x=11/3
x=11/3: 3=11/9
i) 15-x=-45
x=15-(-45)
x=60
Vậy x=60
k) |x-3|=9
Th1: x-3=9
x=9+3=12
Th2: x-3=-9
x=(-9)+3=-6
Vậy x thuộc {12, -6}
l) |x+4|-2=5
|x+4|=5+2
|x+4|=7
Th1: x+4=7
x=7-4=3
Th2: x+4=-7
x=(-7)-4
x=-11
Vậy x thuộc { 3, -11}
m) x-[ 42+(-28)]=-8
x-14=-8
x=(-8)+14
x=6
Vậy x=6
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