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Answer
2^n + 1 is not a prime... because by taking n=3 we will get result as 9 which is divisible by 3
to find whether the number is prime or not we had a technique
lets take an example... lets take 37.... check whether its prime or not
answer
lets start taking 1st prime 2
37 is not even, so 2 is not a factor of 37
now take next prime 3
and 37/3 is 12.3333 so 3 does not divide exactly into 37 either
now next prime 5
and 37 does not end with 0 or 5 so 5 is not a factor of 37
7 is the next prime,
but it is bigger than the square-root of 36, so we can stop now.
So 37 has no factors (except 1 and 37 of course) and therefore
37 is a prime number.
