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Math Help - geometric series

  1. #1
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    geometric series

    Hi all
    Theres this question about a geometric series that i can't figure out.
    It says the second term is 375 and the fifth term is 81 and you have to show that the common ratio of the series is 0.6
    I can't get the right answer :S

    Thanks for the help x
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  2. #2
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    Use the normal geometric progression and substitute the values you have

    Term 2 = 375;
    Term 5 = 81;


    SO now you have to simultaneous equations

    Can be solved
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  3. #3
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    Quote Originally Posted by katiethegreat View Post
    Hi all
    Theres this question about a geometric series that i can't figure out.
    It says the second term is 375 and the fifth term is 81 and you have to show that the common ratio of the series is 0.6
    I can't get the right answer :S

    Thanks for the help x
    you should know the form of a geometric series ...

    a + ar + ar^2 + ar^3 + ...

    2nd term ...

    ar = 375

    5th term ...

    ar^4 = 81


    divide the 5th term by the 2nd term ...

    \frac{ar^4}{ar} = \frac{81}{375}

    r^3 = \frac{27}{125}

    r = \frac{3}{5}
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