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Math Help - How to check if limit does not exist

  1. #1
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    Question How to check if limit does not exist

    So I'm given a function and I have to know what the limit is without drawing a graph. I've learnt that if the denominator is 0 when you sub the number into x, then you have to factorise the function. and if you can't, then the limit does not exist. But then I came across this question: lim x->-2 [[x]].
    [[x]] means the greatest integer function. so in this case x should equal -2? I'm not quite sure how to do this question, but the answer is 'limit does not exist'.
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  2. #2
    Kyo
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    Re: How to check if limit does not exist

    This one may be hard to do without knowing what the greatest integer function means, or seeing the graph visually. A limit only exists when it approaches from both the positive and negative - try to think of this question that way.
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  3. #3
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    Re: How to check if limit does not exist

    I read on the greatest integer function earlier and I think it just means to round down to the smaller integer value.
    so [[5.9]]=5 and [[-2.1]]=-3.
    We're expected to solve these limits algebraically, so graphing wouldn't be the right method.
    Thanks.
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  4. #4
    Kyo
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    Re: How to check if limit does not exist

    Yup, you're right about the GI function. The function brings x to the nearest integer less than x. So, as I hinted at, try to evaluate the limit: lim x->-2(-) and lim x->-2(+), aka: evaluate from the left and the right. If the limit exists, it must be approached from both sides.

    Edit: Even though the solution requires an algebraic solution, looking at the graph may help you realise how to do so.
    Last edited by Kyo; October 28th 2012 at 12:21 AM.
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