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Finding a formula for a difficult sequence

  1. Apr 14, 2009 #1
    1. The problem statement, all variables and given/known data I'm trying to find a formula for 1, 3, 6, 10 ... for n=2,3,4,....







    3. The attempt at a solution
    Well I know that the difference between each number is increasing by 1. For example, the difference between 1 and 3 is 2, between 3 and 6 is 3, between 6 and 10 is 4 and so on. But, I just don't see a formula.
     
  2. jcsd
  3. Apr 14, 2009 #2
    Hi! [tex]a_n = \sum_{ k = 1 }^n k = \frac{ n ( n + 1 ) }{ 2 }[/tex]
     
  4. Apr 14, 2009 #3
    For some reason, I can't read that.
     
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