9999931999-5555571997=9999931996.9999933-5555571996.555557
=(9999934)499.........7-(5555574)499.555557
=...........1499..........7-...........1499.555557
=...................1..........7-.................1.555557
=.....................7-..................7
=................0 chia hết cho 5 vì tận cùng là:0(đpcm)
9999931999-5555571997=9999931996.9999933-5555571996.555557
=(9999934)499.........7-(5555574)499.555557
=...........1499..........7-...........1499.555557
=...................1..........7-.................1.555557
=.....................7-..................7
=................0 chia hết cho 5 vì tận cùng là:0(đpcm)
A=999993^1999-555557^1997
=999993^1996.999997^3-555557^1996.555557
=(999993^4)^499.999997^3-(555557^4)^499.555557
=....1^499. ......7-......1^499....7
=....1. .....7-......1. .....7
=.....7-...7
=....0 chia hết cho 5 vì tận cùng là 0