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Homework Help: Factorials and squares

  1. Sep 27, 2010 #1
    1. The problem statement, all variables and given/known data

    For some natural N the no of positive integers x satisfying the equation
    1! + 2! + 3! + ........ + x! = (N)2 is:
    A)one
    B)two
    C)infinite
    D)none


    3. The attempt at a solution
    I have no idea of how to start.. never came across such problems.
    By trial i have got two values for x being 1 and 3. And the answer is also two . But how do i prove it?

    Please help.(just a starting guidance would be very useful)
     
  2. jcsd
  3. Sep 27, 2010 #2

    fzero

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    Past x=3, the sum of factorials always has a 3 in the last place, s4=33, s5 = 153, ... You need to argue that the square of a natural number cannot end with a 3.
     
  4. Sep 27, 2010 #3
    Or better yet argue n(n-1)(n-2) > n^2 for n>3. If you can prove this you know for N>3 your relation can't hold, since n! > n^2 will be true.
     
  5. Sep 27, 2010 #4
    I found this easier, Thanks. Though i could find the answer by your method too JonF thanks!
     
  6. Sep 27, 2010 #5
    You need to prove this fact if you use his method, “Past x=3, the sum of factorials always has a 3 in the last place"

    n(n-1)(n-2) > n^2 for n>3 Is simple algebra.

    If you expand n(n-1)(n-2) you get (n^2-n)(n-2) = n^3 – 2n^2 –n^2 +2n= n^3 – 3n^2 +2n = n^2(n-3) +2n

    n^2(n-3) +2n is clearly positive if n>3.
     
  7. Sep 28, 2010 #6
    Ok.. got it.
     
  8. Sep 28, 2010 #7
    I'm not overly sure this is useful, as the way I read the original question x and N are two (possibly) different numbers.

    It could be, for instance that 1! + 2! + 3! + 4! + 5! == 162, instead of the 52 you seem to be suggesting. Of course it isn't, but have I misinterpreted what you're saying?
     
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