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Math Help - Geometric Progression with logs

  1. #1
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    Geometric Progression with logs

    Firstly, I know how to find the number of terms of Un=ar^(n-1) using logs. However, I am stuck on a question using the formula; Sn=a(1-r^n)/1-r

    I assumed that the process would be the same with rearanging but I am not getting correct answer.

    The question is;

    A post is being driven into the ground by a mechanical hammer. The distance it is driven by the first blow is 8cm. Each subsequent blow drives the post 90% of the distance of the proceeding blow. If each blow takes 10 seconds how long will it take to drive the post 90% of the distance of the proceeding blow?
    Last edited by peejaygee; March 5th 2014 at 07:56 AM.
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  2. #2
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    Re: Geometric Progression with logs

    Hello,peejaygee!

    No one is answering because your problem makes no sense.
    Please review the wording of the last sentence.


    A post is being driven into the ground by a mechanical hammer.
    The distance it is driven by the first blow is 8cm.
    Each subsequent blow drives the post 90% of the distance of the proceeding blows.
    If each blow takes 10 seconds, how long will it take to drive the post 90% of the distance of the proceeding blow?

    Obviously, the passage in blue is incorrect.

    Please supply the correct distance.
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  3. #3
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    Re: Geometric Progression with logs

    Quote Originally Posted by peejaygee View Post
    Firstly, I know how to find the number of terms of Un=ar^(n-1) using logs. However, I am stuck on a question using the formula; Sn=a(1-r^n)/1-r

    I assumed that the process would be the same with rearanging but I am not getting correct answer.

    The question is;

    A post is being driven into the ground by a mechanical hammer. The distance it is driven by the first blow is 8cm. Each subsequent blow drives the post 90% of the distance of the proceeding blows. If each blow takes 10 seconds how long will it take to drive the post 90% of the distance of the proceeding blow?
    In addition to the sentence soroban marked in blue (which either makes no sense or is trivial), is it 90% of the sum of the preceding blows or 90% of the immediately preceding blow? In one place you use the plural and in another the singular.
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  4. #4
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    Re: Geometric Progression with logs

    That is what was written on my test paper.
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  5. #5
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    Re: Geometric Progression with logs

    It is 90% of the last blow.
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