Question

tìm n thuộc số  tự nhiên, biết:

a) 2ⁿx4=128

b) 3ⁿ⁺¹x9=81

c) 15^n-2=9²: 3⁴

^{x là nhân}

^{nhanh tay 1 tick}

Answer 1

2^n.4=128

2^n=128:4

2^n=32

2^n=2^5

Suy ra n=5

3^n+1.9=81

3^n+1=81:9

3^n+1=9

3^n+1=3^2

Suy ra n=1

15^n-2=9^2:3^4

15^n-2=3^4:3^4

15^n-2=3^0

15^n-2=1

15^n-2=15^0

Suy ra n=2

co gi ko hieu thi hoi nha

Answer 2

a) 2ⁿ . 4 = 128

<=> 2ⁿ . 2² = 2⁷

<=> 2ⁿ⁺² = 2⁷

<=> n+2 = 7

n = 7 - 2

n = 5

b) 3ⁿ⁺¹ . 9 = 81

<=> 3ⁿ⁺¹ . 3² = 3⁴

<=> 3ⁿ⁺¹⁺² = 3⁴

<=> n+3 = 4

n = 4 - 3

n = 1

c) 15^n-2 = 9² : 3⁴

15^n-2 = 1

<=> 15^n-2 = 15⁰

<=> n-2 = 0

n = 0 + 2

n = 2