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Math Help - Geometric Sums

  1. #1
    Junior Member
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    Geometric Sums

    The answer the i keep getting for this question is way off of the one in book. Can anyone tell me what i am doing wrong??

    for each geometric series, find the indicated partial sum

    100+300+900+..... Find S11

    here's what I did:
    Sn=a(r^n-1)/ r-1

    a=100 r=3 n=11

    S11=100(3^11-1)/11-1
    =100(177146)/10
    =17714600/10
    =1771460

    The answer in the back of the book is:
    8857300

    What?
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  2. #2
    MHF Contributor

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    Quote Originally Posted by tmas View Post
    The answer the i keep getting for this question is way off of the one in book. Can anyone tell me what i am doing wrong??

    for each geometric series, find the indicated partial sum

    100+300+900+..... Find S11

    here's what I did:
    Sn=a(r^n-1)/ r-1

    a=100 r=3 n=11

    S11=100(3^11-1)/11-1
    =100(177146)/10
    r- 1= 3- 1= 2 not 11- 1= 10. It is n that is 11, not r.

    =17714600/10
    =1771460

    The answer in the back of the book is:
    8857300

    What?
    Follow Math Help Forum on Facebook and Google+

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