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Homework Help: Sum of problem

  1. Nov 13, 2012 #1
    Find the sum of [itex]\Sigma[/itex] 200 r=5 5r-2

    Sn = n/2 [2a + (n-1)d ]

    I used S 200 and I got about 101400 but then when I verified on my calculator it was 100058, my calculator has the sigma notation for working out the sum of , how do you get 100058?
  2. jcsd
  3. Nov 13, 2012 #2


    Staff: Mentor

    It's difficult to tell what your sum is. Is this it?

    Edit: add parentheses for clarity.
    $$ \sum_{r = 5}^{200} \left(5r - 2\right)$$

    Writing this in LaTeX is not difficult. Click what I wrote to see how I did it.
    Last edited: Nov 13, 2012
  4. Nov 13, 2012 #3

    Ray Vickson

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    Science Advisor
    Homework Helper

    What you wrote coul be reasonably interpreted as
    [tex] \left( \sum_{r=5}^{200} 5 r \right) -2;[/tex] it might be that the OP wants
    [tex] \sum_{r=5}^{200} (5 r - 2), [/tex]
    but who knows?

  5. Nov 13, 2012 #4


    Staff: Mentor

    That's a good point. When I wrote that I was going by a sort of quasi-convention that the summation symbol acts like a giant left parenthesis, and there is an implied right paren to the right of the last symbol. If you want to indicate that something is not part of the sum you put parentheses in to suit.

    Since what I wrote could be interpreted in a couple of ways, I went back and put in some parentheses.

    I wasn't sure what the OP intended, so now I guess we wait until he/she comes back to confirm that our guesses are correct.
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