What is the prime number out of these? Is there an easy way to find that?
(a) (4^7) + 1
(b) (4^8) + 1
(c) (4^9) + 1
(d) (4^10) + 1
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Fermat primes are of the form 2n+1. An easy argument shows that n itself must be a power of 2 -- see Fermat number - Wikipedia, the free encyclopedia. So of your choices only 48+1=216+1 can possibly be prime, and it is.
That's really great thanks for your help
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