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Summation question

  1. Jan 9, 2016 #1
    I know n(n+1)/2 solves from 1.... n by 1's

    Is there a formula where you can add up by 3's?

    Example: 3 + 6 + 9 + 12 ... n
  2. jcsd
  3. Jan 9, 2016 #2


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    Hint: 3+6+9+12=3(1+2+3+4)
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