A = 7.(1 + 7) + 73.(1 + 7) + ... + 735.(1 + 7)
= 7.8 + 73.8 + ... + 735.8
= 8.(7 + 73 + ... + 735) chia hết cho 8
=> A chia 8 dư 0/
A= 7+72+ 73 +74+.....+736= ( 7+72)+ (73+74)+....+ ( 735+736)
= 7(1+7)+73(1+7)+....+735(1+7)
=( 7+73+ ....+735).8:8 dư 0
Vậy A chia 8 dư 0