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Math Help - Help on a convergent sequence limit problem

  1. #1
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    Smile Help on a convergent sequence limit problem



    I tried using the method of doing a limit to infinity which gave me infinity/infinity (an indeterminate) so I used L'hopital's rule and took the derivative of top and bottom. However that still leaves me with that segment. Some help would be nice, thanks.
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  2. #2
    MHF Contributor Drexel28's Avatar
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    Quote Originally Posted by Kitty216 View Post


    I tried using the method of doing a limit to infinity which gave me infinity/infinity (an indeterminate) so I used L'hopital's rule and took the derivative of top and bottom. However that still leaves me with that segment. Some help would be nice, thanks.
    Really? You know L'hopital's rule but you don't know that |k|<1\implies\lim  k^n=0?
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    i do not know that sorry =/
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    MHF Contributor Drexel28's Avatar
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    Quote Originally Posted by Kitty216 View Post
    i do not know that sorry =/
    There's no need to be sorry! It makes sense though, right? If you take an number less than one, then squaring it makes it smaller. Cubing it makes it even smaller, ad infinitum.
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  5. #5
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    Yes you do, though you might not immediately recognize it. What he is saying is that if the absolute value of "k" is less than 1, then the limit as n approaches infinity of k to the n, is 0.

    In your above problem:

    a_{n}=\frac{3^n}{4^n} \Longrightarrow a_{n}=\left(\frac{3}{4}\right)^{n}

    Where k is equal to \frac{3}{4}
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  6. #6
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    oh yea that does make sense since the bottom would keep getting smaller than the top. However my teacher says I have to do it in an algebraic way and show work. Is there really a way with work to show?
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  7. #7
    MHF Contributor Drexel28's Avatar
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    Quote Originally Posted by Kitty216 View Post
    oh yea that does make sense since the bottom would keep getting smaller than the top. However my teacher says I have to do it in an algebraic way and show work. Is there really a way with work to show?
    I mean you can prove it by the definition. Or note that \lim_{n\to\infty}\frac{\left(\frac{3}{4}\right)^{n  +1}}{\left(\frac{3}{4}\right)^n}=\frac{3}{4}<1 so that \sum_{n=1}^{\infty}\left(\frac{3}{4}\right)^n converges which means that \left(\frac{3}{4}\right)^n\to 0
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  8. #8
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    hmm I see... alrighty. Thanks for the help!
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