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)
A= 7(1+7)+ 73(1+7)+....735(1+7)
A= (7+73+....+735).8:8 dư 0
Vậy A chia 8 dư 0