Question

1) Tìm x \(\in\) N biết:

a) 2.(3x - 8)=64:2³

b) 2+4+6+...+2x=210

c) 1+3+5+....+(2x-1)=225

Help me!

Answer 1

a) 2.(3x - 8)=64:2³

vậy : 2.(3x - 8 )=64 : 8

2.(3x - 8) = 8

(3x - 8)= 8:2

(3x - 8)=4

3x = 8+4=12

x=12 : 3

x = 4

b)2+4+6+....+2x=210

vì mỗi số cách nhau 2 đơn vị =>

2+4+6+8+10+12+14+16+18+20+22+24+26+28

vậy 2x=28

x=28:2=14

c)1+3+5+...+(2x-1)=225

Vì mỗi số cách nhau 2 đơn vị=>

1+3+5+7+9+11+13+15+17+19+21+23+25+27+29

vậy (2x - 1)=29

2x=29+1=30

x=30:2=15

like nha

Answer 2

a) 2 . (3x - 8) = 64 : 2³

2 . (3x - 8) = 64 : 8

2. (3x - 8) = 8

3x - 8 = 8 : 2

3x - 8 = 4

3x = 4 + 8

3x = 12

x = 12 : 3

x = 4

b) 2 + 4 + 6 + ... + 2x = 210

(2 + 2x) . [(2x - 2) : 2 + 1] : 2 = 210

[(2 + 2x) : 2]. (x - 1 + 1) : 2 = 210

(1 + x) . x : 2 = 210

x . (x + 1) : 2 = 210

x . (x + 1) = 210 . 2

x . (x + 1) = 420

Ta có: 420 = 42 . 10 = 21 . 2 . 10 = 21 . 20

=> x = 20

c) 1 + 3 + 5 +...+ (2x - 1) = 225
(2x - 1 + 1) . [(2x - 1 - 1) : 2 + 1] : 2 = 225
2x . [(2x - 2) : 2 + 1) : 2 = 225

x . (x - 1 + 1) = 225

x . x = 225

Ta có: 225 = 5 . 45 = 5 . 5 . 9 = 5 . 5 . 3 . 3 = (5 . 3) . (5 . 3) = 15 . 15

=> x = 15