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Math Help - simple solution?

  1. #1
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    simple solution?

    i have a simple question.
    can someone please tell me how to get there?
    I just dont get it.

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  2. #2
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    Also,
    What can i do in this case?
    c(j)c(j + 2)???
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  3. #3
    MHF Contributor

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    Quote Originally Posted by krispiekream View Post
    i have a simple question.
    can someone please tell me how to get there?
    I just dont get it.

    \sum_{j= 0}^\infty \left(0.9\right)^{2j}= \sum_{j=0}^\infty \left(0.9^2\right)^j= \sum_{j= 0}^\infty \left(0.81\right)^j
    is a geometric series with a= 1 and r= 0.81. Use the formula for the sum of a geometric series: \sum_{j=0}^\infty ar^j= \frac{a}{1- r}
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  4. #4
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    thank you..i needed that..
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  5. #5
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    I understand the first step now..
    what about the second step?
    where is 0.9* geometric series come from?



    is it because c(n)=(0.9)^n
    c(0) for n=0, therefore
    c(0)=1
    c(1)=0.9
    c(2)=0.81?
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