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Math Help - limits of sequences with ln(n)

  1. #1
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    limits of sequences with ln(n)

    I have a problem with these sequences: \lim_{x\to\infty} \frac{ln(3n^2+20n+5)}{ln(n^9-3n+12)}
    \lim_{x\to\infty} n ln(n^{2}+1)-2n(lnn)(lnn)^{1/n}
    \lim_{x\to\infty}\frac{1*4*7*...*(3n+1)}{2*5*8*...  *(3n+2)}

    The first one - I don't know how to get rid of the ln(something) function or maybe I'm just doing it in a wrong way. The second one - everything would be great if there wasn't this ln(n)[ln(n)]^{1/n} which I don't know what to do with. The third one - I don't have any idea about this one... I would appreciate your hints or solutions. I've been trying to solve them for so long but I constantly get wrong answers (I used wolfram alpha).
    Last edited by Lisa91; November 11th 2012 at 05:36 PM.
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    Re: limits of sequences with ln(n)

    Quote Originally Posted by Lisa91 View Post
    I have a problem with these sequences: lim n-> infinity [ln(3n^2+20n+5)]/[ln(n^9-3n+12)] lim n-> ininity (n ln(n^2+1)-2n(lnn)(lnn)^(1/n)) lim n> infinity (1*4*7*...*(3n+1))/(2*5*8*...*(3n+2)) The first one - I don't know how to get rid of the ln(something) function or maybe I'm just doing it in a wrong way. The second one - everything would be great if there wasn't this lnn)(lnn)^(1/n) which I don't know what to do with. The third one - I don't have any idea about this one... I would appreciate your hints or solutions. I've been trying to solve them for so long but I constantly get wrong answers (I used wolfram alpha).
    I wonder why you do not learn to post is a readable format?

    LaTex Code is easy to learn.
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    Re: limits of sequences with ln(n)

    Sorry, I've just done it
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    Re: limits of sequences with ln(n)

    That is a lot more readable.

    For the first one, I think you can use L'Hopital's rule.

    - Hollywood
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    Re: limits of sequences with ln(n)

    Are you sure you're making x approach infinity?
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    Re: limits of sequences with ln(n)

    Wolframalpha says that the second one is -\infty and the third one is zero. On the third one, I think it might help to take the log so you have the sum of \log{\frac{3k+1}{3k+2}}.

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    Re: limits of sequences with ln(n)

    Oh, yes, obviously, it's n which approaches infinity. I'm done with the third one but I can't do the first and the second one.
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    Re: limits of sequences with ln(n)

    Hello

    1. Using l'H˘pital:

    \\\lim_{n\to\infty}\dfrac{\ln(3n^2+20n+5)}{\ln(n^9-3n+12)}=\dfrac{\lim_{n\to\infty}\dfrac{6n+20}{3n^2  +20n+5}}{\lim_{n\to\infty}\dfrac{9n^8+3}{n^9-3n+12}}\\\lim_{n\to\infty}\dfrac{\ln(3n^2+20n+5)}{  \ln(n^9-3n+12)}=\lim_{n\to\infty}\dfrac{(6n+20)(n^9-3n+12)}{(9n^8+3)(3n^2+20n+5)}

    You can apply l'H˘pital few more times, but you know that:

    \\\lim_{n\to\infty}\dfrac{\ln(3n^2+20n+5)}{\ln(n^9-3n+12)}=\lim_{n\to\infty}\dfrac{6n^{10}+\ldots}{27  n^{10}+\ldots}\\\lim_{n\to\infty}\dfrac{\ln(3n^2+2  0n+5)}{\ln(n^9-3n+12)}=\dfrac{6}{27}=\dfrac{2}{9}

    2. Making this change of variable:

    \\n=\dfrac{1}{t}\\\lim_{x\to\infty}n\ln(n^2+1)-2n(\ln(n))(\ln(n))^{\frac{1}{n}}=\lim_{t\to0} \dfrac{1}{t}\ln{}\left({}\frac{1}{t^2}{}+1\right)-2\dfrac{1}{t}\ln\left(\frac{1}{t}\right){}\left({}  \ln{}\left({}\frac{1}{t}\right)\right)^t\\=\lim_{t  {}\to{}0}\dfrac{\ln\left(\frac{1}{n^2}+1\right)-2\ln\left(\frac{1}{n}\right)^{t+1}}{t}

    You can continue, just applies L'hopital (because is \dfrac{0}{0}, before that it was \infty-\infty) many times.... The answer is -\infty

    I can't write you the full solution because I don't know how to use \mathrm{\LaTeX} well....

    Greetings
    Last edited by darthjavier; November 13th 2012 at 05:19 AM.
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    Re: limits of sequences with ln(n)

    Quote Originally Posted by Plato View Post
    I wonder why you do not learn to post is a readable format?


    LaTex Code is easy to learn.
    Are you serious?

    Quote Originally Posted by Prove It View Post
    Are you sure you're making x approach infinity?
    lol I'm sure he wanted to type n.
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    Re: limits of sequences with ln(n)

    Quote Originally Posted by darthjavier View Post
    Are you serious?
    Yes! I was quite serious.
    If you look at the OP before it was edited, you would why.
    As you can see it was made readable.

    And LaTeX is easy to learn.
    If I had my way, all regular posters should be required to us LaTeX code.
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  11. #11
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    Re: limits of sequences with ln(n)

    Quote Originally Posted by Plato View Post
    Yes! I was quite serious.
    If you look at the OP before it was edited, you would why.
    As you can see it was made readable.

    And LaTeX is easy to learn.
    If I had my way, all regular posters should be required to us LaTeX code.
    Yes, I'm agree with you. I discovered recently \mathrm{\LaTeX}, and I try to practice every day because is very useful.

    But I mean that \mathrm{\LaTeX} is difficult to learn, especially on this page. When I post it's too annoying see the equations combined with the HTML code. I can't see the HTML when I'm editing my \mathrm{\LaTeX} code, I don't know why. In other pages I use an editor (in my case use TextMaker) and I only had to copy-paste. But here I have to write everything on one line, and it seems that LaTeX doesn't work well here, sometimes there are mistakes, I have to write "{}" all the time. I don't know if I'm the only one with this problems.
    Last edited by darthjavier; November 13th 2012 at 05:19 AM.
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    Re: limits of sequences with ln(n)

    Thank you very much, guys. I am wondering whether I am allowed to apply the de L'Hospital rule because we have a sequence, not a function here. I think I need to proof that let's say  ln(n^2+20n+5) and  ln(n^9-3n+12) are differentiable. Is there anyone who's studying math?
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