Question

[Mèo cute]

Cho A=11^9+11^8+......+11+1(CMR A chia hết cho 5)

Answer 1

[minhduc]

 ta co' tinh chat cua luy thua cua 11 nhu sau:

So cuoi cung cua 11^x luon = 1. 
Tu` do' ta de dang thay':A= 11^9+11^8+...+11+1 cac so hang deu co so tan cung = 1 va co 10 so hang do do' so' tan cung cua tong?

nay` la` 0. Vay A chia het cho 5. 

Answer 2

[KAl(SO4)2·12H2O]

Ta có:

A = (11⁹ + 11⁸ + 11⁷ + 11⁶ + 11⁵) + (11⁴ +11³ + 11² + 11 + 1)

A =            Chia hết cho 5              +           Chia hết cho 5

=> A chia hết cho 5