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Math Help - Domain of function

  1. #1
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    Algebra question

    1/ x^2-10x - 8
    I know I have to solve for zero and factor it but when I do it just comes out 1, -5 but the awnser is -1 ,5 ... I just dont understand why ? Sorry math is the subject I struggle with most .Thank you
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  2. #2
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    Quote Originally Posted by starbrite
    1/ x^2-10x - 8
    I know I have to solve for zero and factor it but when I do it just comes out 1, -5 but the awnser is -1 ,5 ... I just dont understand why ? Sorry math is the subject I struggle with most .Thank you
    Where is the equation!

    \frac{1}{x^2-10x-8}=??
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  3. #3
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    oh sorry its f(x)
    I have to find the domain of the function . Thanks
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  4. #4
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    Quote Originally Posted by starbrite
    oh sorry its f(x)
    I have to find the domain of the function . Thanks
    It math course they teach you whenever they write a function they always assume all possible values that 'x' can take since you have a fraction,
    \frac{1}{x^2-10x-8}
    It can be any value EXCEPT when the denominator is zero.
    Thus,
    x^2-10x-8\not =0
    Since this is not factorable use quadradic formula,
    x=\frac{10\pm \sqrt{100+64}}{2\cdot 1}
    Simplfy,
    x=\frac{10\pm 2\sqrt{41}}{2}
    Thus,
    x=5\pm \sqrt{41}
    ---
    To make things clearly, let me explain,
    \sqrt{100+64}=\sqrt{164}=\sqrt{4\cdot 41}=2\sqrt{41}

    And, in the final step since the numbers shared a common factor namely 2 I divided it out. Important "you cannot cancel out of a sum" meaning you cannot cancel a 2 with the 10 and leave the other 2. You must to cancel both of them whenever you deal with add/subt.
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  5. #5
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    oh ok thank u so much
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