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Math Help - Sequences and series..

  1. #1
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    Sequences and series..

    So, how do I solve this to find the value of n ?

    Un = (-1)^n (n / n + 4) and I know that Un = 7 / 9 ?

    So do you times (-1)^n and ( n / n + 4) ??

    Someone help me out here, with explanation please!
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  2. #2
    Senior Member bkarpuz's Avatar
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    Quote Originally Posted by Mathematix View Post
    So, how do I solve this to find the value of n ?

    Un = (-1)^n (n / n + 4) and I know that Un = 7 / 9 ?

    So do you times (-1)^n and ( n / n + 4) ??

    Someone help me out here, with explanation please!
    What is the question exactly?
    I did not understand anything about it.
    You have u_{n}=(-1)^{n}\frac{n}{n+4} and what do you want to solve?
    It seems u_{n} is already in an explicit form.
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  3. #3
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    ^ From that, I need to know how to find the value of n.
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  4. #4
    Senior Member bkarpuz's Avatar
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    Exclamation

    Quote Originally Posted by Mathematix View Post
    ^ From that, I need to know how to find the value of n.
    Are you asking for which n value u_{n}=\frac{7}{9} holds?

    If so, n=14.
    To find the solution, first consider n=2k then n=2k-1 for k\in\mathbb{N}.
    In the first case, you will find k=7 ( n=14) and in the second one k=-\frac{3}{8}\not\in\mathbb{N}.
    Last edited by bkarpuz; September 27th 2008 at 02:12 PM.
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  5. #5
    is up to his old tricks again! Jhevon's Avatar
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    you are somewhat vague, Mathematix, but if bkarpuz's last post accurately said what you want, then you must solve \frac n{n + 4} = \frac 79 for n. and note that n must be even. so if you get an odd number, you are wrong
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