Ta có:
\(A=999993^{1999}-555557^{1997}\)
\(A=999993^{1998}.999993-555557^{1996}.555557\)
\(A=\left(999993^2\right)^{999}.999993-\left(555557^2\right)^{998}.555557\)
\(A=\overline{\left(.....9\right)}^{999}.999993-\overline{\left(.....1\right)}.555557\)
\(A=\overline{\left(.....7\right)}-\overline{\left(.....7\right)}\)
\(A=\overline{\left(.....0\right)}\)
Vì A có tận cùng là 0
\(\Rightarrow A⋮5\) (Đpcm)
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