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Math Help - geometric series

  1. #1
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    geometric series

    Hey all

    I was stuck with this question...
    A geometric series has second term 375 and fifth term 81 .
    (a) (i) Show that the common ratio of the series is 0.6

    I got as far as figuring out that ar= 375 and ar^4 = 81 ....but then i got confused.

    Thanks xx
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  2. #2
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    375 * .6^3 = 81
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  3. #3
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    Quote Originally Posted by katiethegreat View Post
    Hey all

    I was stuck with this question...
    A geometric series has second term 375 and fifth term 81 .
    (a) (i) Show that the common ratio of the series is 0.6

    I got as far as figuring out that ar= 375 and ar^4 = 81 ....but then i got confused.

    Thanks xx
    Divide them

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

    Working is in the spoiler, but remember you'll only be cheating yourself if you don't understand...

    Spoiler:
    r = \sqrt[3]{\frac{81}{375}} = \sqrt[3]{\frac{27}{125}} = \sqrt[3]{\frac{3^3}{5^3}} = \frac{3}{5}
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