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Math Help - Prime Numbers

  1. #1
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    Prime Numbers

    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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  2. #2
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    Re: Prime Numbers

    Hi Kristen1...
    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.
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  3. #3
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    Re: Prime Numbers

    That's really great thanks for your help
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