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Need help solving a combination for n

  1. Nov 27, 2004 #1
    Need help solving a combination for "n"

    I've been unable to solve the following combination for "n":

    nC3 =10

    I got this equation : n³ - 3n² + 2n -30 = 0

    But I can't find a common factor of 30 that can be plugged into the equation to equal zero. Am I solving this incorrectly perhaps? Should I be doing something different?
     
  2. jcsd
  3. Nov 27, 2004 #2

    Gokul43201

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    You mistake is with the 30. How did you get that ?
     
  4. Nov 27, 2004 #3
    I ended up with n³ - 3n² + 2n / 3! = 10 so I just multiplied the 3 by the 10 to get rid of the denominator. Come to think of it 3!=6 so maybe it should be 60 instead right?
     
  5. Nov 27, 2004 #4
    Yeah perfectly right
    what you get is

    n^3 - 3n^2 + 2n - 60 = 0

    Solving for n
    n = 5 or -1

    neglecting -1

    The solution is n=5

    5C3 = 10
     
  6. Nov 27, 2004 #5

    Gokul43201

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    That's right.

    Prasanna, the policy here is that we do not solve the problem for the poster, but give suggestions or point out errors. That way, the poster will actually learn how to go about problem solving.
     
  7. Nov 27, 2004 #6
    Thank you for the help!
     
  8. Nov 28, 2004 #7
    Oops!! Sorry I did not know that!!
    I will take care hereafter!
     
  9. Nov 28, 2004 #8
    Are you sure -1 is a solution? If you plug the value into the original cubic, you get -66, not 0.
     
  10. Nov 29, 2004 #9
    Yeah right
    n = -1 is indeed wrong
    n = -1 + sqrt(11)i
    or
    n = -1 - sqrt(11)i
     
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