999993^1999 = 999993^1996 . 999993^3 = (999993^4)^499 . 999993^3 = ...1 . ...7 = ...7
555557^1997 = 555557^1996 . 555557 = (555557^4)^499 . 555557 = ...1 . ...7 = ...7
999993^1999 - 555557^1997 = ...7 - ...7 = ...0
Chữ số tận cùng là 0 nên chia hết cho 5
\(A=999993^{1999}-555557^{1997}\)
CMR A chia het cho 5