`A=999993^{1999}-555557^{1997}`
`⇒ A=999993^{1998}.999993-555557^{1996}.555557`
$⇒ A=\left(999993^2\right)^{999}.999993-\left(555557^2\right)^{998}.555557$
$⇒ A=\left(....9\right)^{999}.999993-\left(....1\right).555557$
`⇒ A = (....7) - (....7)`
`⇒ A = (....0)`
Vì `A` có tận cùng là `0`
`⇒ A \vdots 5` `(Đpcm)`