Results 1 to 4 of 4

Math Help - Help with arithmetic series.

  1. #1
    Newbie
    Joined
    Dec 2010
    Posts
    2

    Help with arithmetic series.

    Hi, I'm taking AS Maths Core 1 in January. I am stuck on a question in the text book.
    It goes like this:
    Calculate:

    5
    Sigma r(r+1)
    r = 0

    The answer in the text book is 112 but I can't get that. I get 70 as an answer. I would really appreciate any help with how to solve this particular problem.]

    Thanks in advance,

    j
    Last edited by mr fantastic; December 27th 2010 at 06:30 PM. Reason: Title.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor

    Joined
    Aug 2006
    Posts
    18,605
    Thanks
    1574
    Awards
    1
    The answer to the problem you posted is 70.

    So check the problem again. Is it correct?

    If so, the given answer is wrong. It goes with \sum\limits_{r = 0}^6 {r\left( {r + 1} \right)} .
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Senior Member
    Joined
    Dec 2010
    Posts
    470
    The easiest way to do this summation is by using combinations:

    \sum_{r=0}^5 r(r+1) = \sum_{r=0}^5 2\binom{r+1}{2} = 2\binom{7}{3} = 2\frac{7\times 6 \times 5}{3!} = 70

    You can always check by a direct sum
    0*1 + 1*2 + 2*3 + 3*4 + 4*5 + 5*6
    = 0 + 2 + 6 + 12 + 20 + 30 = 70

    so you are absolutely correct

    However,
    \sum_{r=0}^6 r(r+1) = 2\binom{8}{3} = 2\frac{8\times 7 \times 6}{3!} = 112
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Newbie
    Joined
    Dec 2010
    Posts
    2
    Thanks so much. It had my head chewed up all day. It is a misprint in the book. I checked so many things except for the upper limit.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Arithmetic Series Help
    Posted in the Algebra Forum
    Replies: 3
    Last Post: March 30th 2010, 05:16 AM
  2. Arithmetic Progression or Arithmetic Series Problem
    Posted in the Math Topics Forum
    Replies: 1
    Last Post: October 8th 2009, 12:36 AM
  3. Arithmetic Series
    Posted in the Algebra Forum
    Replies: 2
    Last Post: July 17th 2009, 11:26 AM
  4. Arithmetic series
    Posted in the Algebra Forum
    Replies: 2
    Last Post: November 19th 2008, 05:49 AM
  5. arithmetic series
    Posted in the Algebra Forum
    Replies: 1
    Last Post: September 18th 2008, 05:36 PM

Search Tags


/mathhelpforum @mathhelpforum