\(2^n-1⋮7\)
\(\Leftrightarrow2^n-1\inƯ\left(7\right)\)
Mà \(n\in N\Leftrightarrow2^n-1\in N\)
\(\Leftrightarrow\left[{}\begin{matrix}2^n-1=1\\2^n-1=7\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}2^n=2\\2^n=8\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}n=1\\n=3\end{matrix}\right.\)
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