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Homework Help: Very Important Help Series

  1. Apr 8, 2005 #1
    I have a question is sigma notation

    92
    E (-7)^i+7
    i=7

    Is the number of terms in this series 99? Or

    86, 89, or 96 im so confused PLZ HELP IMMEDIATELY

    I think it is 99 because 92+7=99 but im not sure
     
    Last edited: Apr 8, 2005
  2. jcsd
  3. Apr 8, 2005 #2

    dextercioby

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    Please write it more clearly.Is that "i" [itex] \sqrt{-1} [/itex]...?

    Use LaTex.

    Daniel.
     
  4. Apr 8, 2005 #3
    no its

    (-7) to the exponent (i+7)

    hurry help
     
  5. Apr 8, 2005 #4

    dextercioby

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    So that "i" wanders through "n"...?

    Daniel.

    P.S.If so,then is 99 a natural power of 7...?
     
  6. Apr 8, 2005 #5

    shmoe

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    Is this your sum?

    [tex]\sum_{i=7}^{92}(-7)^{i-7}[/tex]

    You have a term for each number 7, 8, 9, ..., 91, 92. How many numbers on this list?
     
  7. Apr 8, 2005 #6

    dextercioby

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    OMG,i didn't understand the question...No wonder the ballooney...:eek: :yuck:

    So it was that simple...

    Daniel.
     
  8. Apr 9, 2005 #7
    I believe there are 86 in the terms. you take 92-7+1=86
     
  9. Apr 10, 2005 #8

    HallsofIvy

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    Looks like a simple geometric sum to me.
    [tex]\sum_{i=7}^{92}(-7)^{i-7}[/tex]

    Let n= i- 7 so that when i= 7, n=0 and when i= 92, n= 92-7= 85 (as Gieuseppe said).

    The sum is the same as [tex]\sum_{n=0}^{85}(-7)^{n}[/tex].

    Can you do that? (There is a simple formula for geometric sums.)
     
  10. Apr 11, 2005 #9
    [tex]\sum_{i=7}^{92}(-7)^{i+7}[/tex]

    the exponent is i plus 7 not minus. Also the answer is 86 not 85 but I dont know how to get the answer I tried so many times.
     
  11. Apr 11, 2005 #10

    shmoe

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    Can you answer the question how many numbers are on the list 7, 8, 9, ..., 92?

    How about we subtract 6 from each number. The list:

    7, 8, 9, ..., 92

    has the same number of items as:

    1, 2, 3, ..., 86



    In general if you have a sum whose index ranges from a to b, you have b-a+1 terms.
     
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