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anonymous

  • one year ago

If you want to examine if a integer is a prime number can you try with dividing it by the prime numbers 2 , 3, 5 etc . What is the largest prime number that there is reason to try ? Determine if any of the numbers 171, 203 , 211, 567 or 7669 is prime and factorize them otherwise.

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  1. IrishBoy123
    • one year ago
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    the number's square root is a cut off point

  2. welshfella
    • one year ago
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    A short cut to see if a number is divisible 3 is to add the digits up until you have 1 didigte If this digit is 3 , 6 or 9 then t is divisible by 3.

  3. welshfella
    • one year ago
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    so from this you can see that 171 and 567 are not prime

  4. welshfella
    • one year ago
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    square root of 203 is 14.25 so you keep dividing by prime numbers up to and including 13.

  5. zzr0ck3r
    • one year ago
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    It is sort of fun to think about why the square root thing works.

  6. anonymous
    • one year ago
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    @@zzr0ck3r could you explain why that works? is it because everything after the squareroot is like testing the dividers all over again?

  7. welshfella
    • one year ago
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    I think it works because if there is an integer quotient you would have found it already when dividing by the lower primes.

  8. welshfella
    • one year ago
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    - yes - it is like testing the dividers all over again

  9. imqwerty
    • one year ago
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    well lets take any number say x the biggest factor of any number can be the square root of the number itself :) when u prime factorize x u will get prime numbers as its factors so the prime factors of any number x are always smaller than or equal to root{x} if x is a prime number then there will be no factors but if x is composite then the factors it will have will always be less than or equal to root(x) so to check if x is prime we find root x nd then try dividing x by all prime numbers less then rootx

  10. anonymous
    • one year ago
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    so if Im gonna see if 7669 is a prime, how do i do? the sqrt of 7669 is 87.6, then still I gotta know the primes upp to that

  11. anonymous
    • one year ago
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    @zzr0ck3r , @welshfella , @imqwerty

  12. zzr0ck3r
    • one year ago
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    yes

  13. welshfella
    • one year ago
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    yes 2,3,5,7 etc

  14. anonymous
    • one year ago
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    up to 83

  15. welshfella
    • one year ago
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    thats correct because square root of 7669 is 87.57

  16. welshfella
    • one year ago
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    oh you found that already - sorry

  17. welshfella
    • one year ago
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    is 211 a prime?

  18. anonymous
    • one year ago
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    no, 3*3*19

  19. welshfella
    • one year ago
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    that is 171

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