Question

[nguyễn hương Xuân]

Cho a = \(2+2^2+2^3+.....+2^{59}+2^{ }^{60}\)

a)Achia hết cho 2

B)a chia hết cho 3

C)a chia hết cho 7

hãy chứng minh a , b , c

Answer 1

[Minh Hiếu]

a) \(A=2\left(1+2+2^2+...+2^{59}\right)⋮2\)

b) \(A=2\left(1+2\right)+2^3\left(1+2\right)+...+2^{59}\left(1+2\right)\)

\(=3\left(2+2^3+...+2^{59}\right)⋮3\)

c) \(A=2\left(1+2+2^2\right)+2^5\left(1+2+2^2\right)+...+2^{58}\left(1+2+2^2\right)\)

\(=7\left(2+2^5+...+2^{58}\right)⋮7\)

Answer 2

[Kiều Vũ Linh]

a) A = 2 + 2² + 2³ + ... + 2⁵⁹ + 2⁶⁰

= 2.(1 + 2 + 2² + ... + 2⁵⁸ + 2⁵⁹) 2

Vậy A ⋮ 2

b) A = 2 + 2² + 2³ + ... + 2⁵⁹ + 2⁶⁰

= (2 + 2²) + (2³ + 2⁴) + ... + (2⁵⁹ + 2⁶⁰)

= 2.(1 + 2) + 2³.(1 + 2) + ... + 2⁵⁹.(1 + 2)

= 2.3 + 2³.3 + ... + 2⁵⁹.3

= 3.(2 + 2³ + ... + 2⁵⁹) ⋮ 3

Vậy A ⋮ 3

c) A = 2 + 2² + 2³ + 2⁴ + 2⁵ + 2⁶ + ... + 2⁵⁸ + 2⁵⁹ + 2⁶⁰

= (2 + 2² + 2³) + (2⁴ + 2⁵ + 2⁶) + ... + (2⁵⁸ + 2⁵⁹ + 2⁶⁰)

= 2.(1 + 2 + 2²) + 2⁴.(1 + 2 + 2²) + ... + 2⁵⁸.(1 + 2 + 2²)

= 2.7 + 2⁴.7 + ... + 2⁵⁸.7

= 7.(2 + 2⁴ + ... + 2⁵⁸) ⋮ 7

Vậy A ⋮ 7