Determine all positive integer n for which 2n+1 is divisible by 3.
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Solution
2n+1 is divisible by 3. ⇒(3−1)n+1 is divisible by 3 ⇒(−1)n+1 is divisible by 3 ⇒nisodd Set of all odd natural numbers in the set of all those positive integer for which 2n+1 is divisible by 3.