The smallest value of natural number n, for which 2n + 1 is not a prime number is -

Question: The smallest value of natural number n, for which 2n + 1 is not a prime number is -

Solution:
Putting n = 1, 2, 3, 4, ........ respectively in (2n + 1)

We get,
n = 1; (2 × 1) + 1 = 3
n = 2; (2 × 2) + 1 = 5
n = 3; (2 × 3) + 1 = 7
n = 4; (2 × 4) + 1 = 9

Where 3, 5, 7 are prime numbers.
But 9 is not a prime number.

∴ The smallest value of natural number n, for which 2n + 1 is not a prime number is n = 4.