S = 2 + 22 + 23 + 24 + ..... + 2100
S = (2 + 22 + 23 + 24 + 25) + ....... + (296 + 297 + 298 + 299 + 2100)
S = (2.1 + 2.2 + 2.4 + 2.8 + 2.16) + ..... + (296.1 + 296.2 + 296.4 + 296.8 + 296.16)
S = 2.(1+2+4+8+16) + .... + 296.(1+2+4+8+16)
S = (1+2+4+8+16).(2 + 26 + ..... + 296)
S = 31 . (2 + 26 + ..... + 296)
Vậy S chia hết cho 31
D = 22000 + 22002
D = 21990.10 + 21990 . 212
D = 21990.(212 + 212)
D = 21990 . 5120
Vậy D chia hết cho 5120