Question

[Nguyễn Demon]

cho bieu thuc M= 5+5²+5³+.....+5⁸⁰chung to rang 

a, M chia het cho 6

b, M khong phai la so chinh phuong

Answer 1

[Nguyễn Phương Uyên]

a, M = 5 + 5^2 + 5^3 + ... + 5^80

M = (5 + 5^2) + (5^3 + 5^4) + ... + (5^79 + 5^80)

M = 30 + 30.5^2 + ... + 30.5^78

M = 30(1 + 5^2 + ... + 5^78) vi 30 ⋮ 6

=> M ⋮ 6

Answer 2

[Nguyễn Việt Hoàng]

M = 5 + 5² + 5³ + ... + 5⁸⁰

M = (5 + 5²) + (5³ + 5⁴) + ... + (5⁷⁹ + 5⁸⁰)

M = 30 + 30.5²+ ... + 30.5⁷⁸

M = 30(1 + 5² + ... + 5⁷⁸) vì 30 ⋮ 6

=> M ⋮ 6