Lời giải:
Ta có: \(19^2=361\equiv 10\pmod {27}\)
\(\Rightarrow 19^3=19^2.19\equiv 10.19\equiv 1\pmod {27}\)
Suy ra:
\(7^3=19\pmod {27}\Rightarrow 7^{9}\equiv 19^3\equiv 1\pmod {27}\)
Vậy \(19^3\equiv 7^9\equiv 1\pmod {27}\)
Khi đó:
\(19^{2008}+7^{2008}=(19^{3})^{669}.19+(7^9)^{223}.7\)
\(\equiv 1^{669}.19+1^{223}.7\equiv 19+7\equiv 26\pmod {27}\)
Vậy \(19^{2008}+7^{2008}\) chia $27$ dư $26$
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