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Math Help - Inequalities

  1. #1
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    Inequalities

    which one (s) of the following inequalities are then known to be x^2-77> 0
    a) x>-5
    b)x>5
    c)lxl>5

    this is an old exam i train and the answer shall be c) but idk how they do it.. My progress is that i solve x>sqrt(77)
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  2. #2
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    Re: Inequalities

    I don't understand the question (probably a language issue). If X is known to be Y, then X is Y and this fact is known. None of the inequalities from the answers literally are x^2 - 77 > 0. Try rephrasing your question using the word "implies."
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  3. #3
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    Re: Inequalities

    Quote Originally Posted by emakarov View Post
    I don't understand the question (probably a language issue). If X is known to be Y, then X is Y and this fact is known. None of the inequalities from the answers literally are x^2 - 77 > 0. Try rephrasing your question using the word "implies."
    I Will do it soon i used google translate:P
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  4. #4
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    Re: Inequalities

    If x^2- 77> 0 then x^2> 77 so, taking the square root of both sides, either x> \sqrt{77} or x< -\sqrt{77} which is the same as |x|> \sqrt{77}. Now, \sqrt{77}= 8.77 so that |x|> 8.77. Of course, if |x|> 8.77 then it is certainly true that |x|> 5. Although that is not the "best" inequality, it is the only one of the given answers that is true.
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  5. #5
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    Re: Inequalities

    Should not b also be true x>5? Idk i kinda think it was a bad question they had on a old exam
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  6. #6
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    Re: Inequalities

    Quote Originally Posted by Petrus View Post
    Should not b also be true x>5? Idk i kinda think it was a bad question they had on a old exam
    No.
    Reread reply #4 carefully.
    x=-9 is a solution for x^2-77>0. BUT -9\not>5.
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