+ Nếu n = 3k thì 2n - 1 = 23k - 1 = 8k - 1
Có: \(8\equiv1\left(mod7\right)\Rightarrow8^k\equiv1\left(mod7\right)\Rightarrow8^k-1\equiv0\left(mod7\right)\)
(TM)
+Nếu n = 3k+1 thì 2n - 1 = 23k+1 - 1 = 8k.2 - 1
Vì \(8^k\equiv1\left(mod7\right)\)nên \(8^k.2\equiv2\left(mod7\right)\Rightarrow8^k.2-1\equiv1\left(mod7\right)\)(ko TM)
+ Nếu n = 3k+2 thì 2n - 1 = 23k+2 - 1 = 8k.4 - 1
Vì 8k \(\equiv\) 1 (mod7) nên 8k.4 \(\equiv\) 4 (mod7) => 8k.4 - 1\(\equiv\) 3 (mod7) (ko TM)
Vậy n = 3k (k thuộc N*) thỏa mãn đề