Question

chung to rang:

A= 2 + 2^2 + 2^3 + 2^4 + ...+ 2^90

Answer 1

chia het cho 21 nua nhe

Answer 2

chia het cho may ban

Answer 3

chia het cho 21 bn Nguyễn Phúc Thịnh

Answer 4

oke

ta có 

A = 2+2^2+2^3+2^4+...+2^90

= ( 2+2^2+2^3+2^4+2^5+2^6)+(2^7+2^8+2^9+2^10+2^11+2^12)+...+(2^85+2^86+2^87+2^88+2^89+2^90)

=1.(2+2^2+2^3+2^4+2^5+2^6)+2^6.( 2+2^2+2^3+2^4+2^5+2^6)+...+2^84.( 2+2^2+2^3+2^4+2^5+2^6)

=( 2+2^2+2^3+2^4+2^5+2^6).(1+2^6+2^12+...+2^84)

=126.(1+2^6+2^12+...+2^84)

=6x21.(1+2^6+2^12+...+2^84) chia hết cho 21

Vậy A chia hết cho 21