Bài 1 : tìm số tự nhiên x, biết:
8x + 15 =87
10+9x =109
7x – 8 =713
951 – 4x=83
98:x+25=39
802+x:3=919
235-x:8=123
1308+x:75=34
9x-7x=2018
23x-x=88
Bài 2: Tìm số tự nhiên x, biết:
3.(x+25)=60
(18+x).9=270
2.(37-x)=42
(109-x).8=96
4.(x-29)=200
(x-27).7=84
94:(x+11)=2
(15+x):14=5
75:(39-x)=15
(918-x):5=25
129:(x-7)=43
(x-28):6=12
Bài 1:
a) 8x+15=87
8x=87-15
8x=72
x=72:8
x=9
b) 10+9x=109
9x=109-10
9x=99
x=99:9
x=11
c) 7x-8=713
7x=713+8
7x=721
x=721:7
x=103
d) 951-4x=83
4x=951-83
4x=868
x=217
e) 98:x+25=39
98:x=39-25
98:x=14
x=98:14
x=7
f) 802+x:3=919
x:3=919-802
x:3=117
x=117.3
x=351
g) 235-x:8=123
x:8=235-123
x:8=112
x=112.8
x=896
h) 1308+x:75=34
x:75=1308-34
x:75=1274
x=1274.75
x=95550
i) 9x-7x=2018
2x=2018
x=2018:2
x=1009
k) 23x-x=88
22x=88
x=88:22
x=4
Bài 2:
a) 3.(x+25)=60
x+25=60:3
x+25=20
x=20-25
x=-5
b) (18+x).9=270
18+x=270:9
18+x=30
x=30-18
x=12
c) 2.(37-x)=42
37-x=42:2
37-x=21
x=37-21
x=16
d) (109-x).8=96
109-x=96:8
109-x=12
x=109-12
x=97
e) 4.(x-29)=200
x-29=200:4
x-29=50
x=50+29
x=79
f) (x-27).7=84
x-27=84:7
x-27=12
x=12+27
x=39
g) 94:(x+11)=2
x+11=94:2
x+11=47
x=47-11
x=36
h) (15+x):14=5
15+x=5.14
15+x=70
x=70-15
x=55
Đáp án:
Bài 1.
1. 8x +15 = 87
⇔ 8x = 87-15
⇔ 8x = 72
⇔ x = 72 : 8
⇔ x = 9
2. 10 + 9x = 109
⇔ 9x = 109 – 10
⇔ 9x = 99
⇔ x = 99 : 9
⇔ x = 11
3. 7x – 8 = 713
⇔ 7x = 713+8
⇔ 7x = 721
⇔ x = 721 : 7
⇔ x = 103
4. 951 – 4x = 83
⇔ 4x = 951 – 83
⇔ 4x=868
⇔ x=217
5. 98 : x + 25 = 39
⇔ 98 : x = 39-25
⇔ 98 : x = 14
⇔ x = 98 : 14
⇔ x=7
6. 802 + x : 3 = 919
⇔ x : 3 = 919 – 802
⇔ x : 3 = 117
⇔ x = 117 × 3
⇔ x = 351
7. 235 – x : 8 = 123
⇔ x : 8 = 235 – 123
⇔ x : 8 = 112
⇔ x = 112×8
⇔ x = 896
8. 1308 + x : 75 = 34
⇔ x : 75 = 1308 – 34
⇔ x : 75 = 1274
⇔ x = 1274× 75
⇔ x = 95550
9. 9x – 7x = 2018
⇔ (9 – 7)x = 2018
⇔ 2x = 2018
⇔ x = 2018 : 2
⇔ x=1009
10. 23x – x = 88
⇔ (23 – 1)x = 88
⇔ x =88 : 22
⇔ x = 4
Bài 2
1. 3.(x+25)=60
⇔ x + 25 = 60 : 3
⇔ x + 25 = 20
⇔ x = 20-25
⇔ x=-5
2. (18 + x ).9 = 270
⇔18 + x = 270:9
⇔18 + x = 30
⇔x = 30 – 18
⇔ x = 12
3. 2.(37-x) = 42
⇔ 37 – x = 42 : 2
⇔ 37 – x = 21
⇔ x = 37-21
⇔ x = 16
4. (109 – x).8 = 96
⇔ 109 – x = 96:8
⇔ 109 – x = 12
⇔ x = 109-12
⇔ x = 97
5. 4.(x – 29) = 200
⇔ x – 29 = 200 : 4
⇔ x – 29 = 50
⇔ x = 50 + 29
⇔ x = 79
6. (x-27).7 = 84
⇔ x – 27 =84 : 7
⇔ x – 27 = 12
⇔ x = 12 + 27
⇔ x = 39
7. 94 : (x + 11) = 2
⇔ x + 11 = 94 : 2
⇔ x + 11 = 47
⇔ x = 47-11
⇔ x = 36
8. (15 + x ) : 14 = 5
⇔ 15 + x = 5.14
⇔ 15 + x = 70
⇔ x = 70 – 15
⇔ x = 55
9. 75 : (39 – x) = 15
⇔ 39 – x = 75 : 15
⇔ 39 – x = 5
⇔ x = 39 – 5
⇔ x = 34
10. (918 – x) : 5 = 25
⇔ 918 – x = 25.5
⇔ 918 – x = 125
⇔ x = 918 – 125
⇔ x = 793
11. 129 : (x – 7) = 43
⇔ x – 7 = 129 : 43
⇔ x – 7 = 3
⇔ x = 10
12. (x – 28) : 6 = 12
⇔ x – 28 = 12 . 6
⇔ x – 28 = 72
⇔ x = 72 + 28
⇔ x = 100