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Math Help - Find the maximum

  1. #1
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    Find the maximum

    f(x)=\frac {x-5}{x^2-5x+1992}
    For which x does it have its maximum?
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  2. #2
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    Quote Originally Posted by james_bond View Post
    f(x)=\frac {x-5}{x^2-5x+1992}
    For which x does it have its maximum?
    Have you tried equating the derivative to zero and testing the nature of the resulting solutions?
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  3. #3
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    No but unfortunately I need a solution without using calculus. Maybe with using some well-known inequalities (AM-GM, Cauchy, etc.) and some tricks...
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  4. #4
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    Quote Originally Posted by james_bond View Post
    No but unfortunately I need a solution without using calculus. Maybe with using some well-known inequalities (AM-GM, Cauchy, etc.) and some tricks...
    Although you need to find a solution without using calculus, that doesnít mean you canít use calculus to find what the maximum value might be. The problem should be easier if you know what the maximum value is.

    Using calculus, I found that the maximum value occurs at x=5+\sqrt{1992}. So the problem is to use non-calculus methods to prove that \frac{x-5}{x^2-5x+1992}\leq\mathrm{f}\left(5+\sqrt{1992}\right) for all x\in\mathbb{R}.

    \mathrm{f}\left(5+\sqrt{1992}\right) should simplify to \frac{1}{5+2\sqrt{1992}}. So the inequality becomes

    \color{white}.\quad. \frac{x-5}{x^2-5x+1992}\ \leq\ \frac{1}{5+2\sqrt{1992}}

    \Leftrightarrow\ (x-5)(5+2\sqrt{1992})\ \leq\ x^2-5x+1992 (note that x^2-5x+1992>0 for all x)

    \Leftrightarrow\ 5x-25+2x\sqrt{1992}-10\sqrt{1992}\ \leq\ x^2-5x+1992

    \Leftrightarrow\ 0\ \leq\ x^2-2(5+\sqrt{1992})x+25+10\sqrt{1992}+1992\ =\ \left(x-[5+\sqrt{1992}]\right)^2

    QED.
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  5. #5
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    The simple answer was:
    f(x)=\frac {x-5}{x^2-5x+1992}=\frac{1}{x+\frac{1992}{x-5}}=\frac{1}{x-5+\frac{1992}{x-5}+5}\le\frac{1}{4\sqrt{498}+5} by AM-GM
    Equality occurs when x-5=\frac{1992}{x-5}\implies x=2\sqrt{498}+5
    Last edited by james_bond; April 17th 2008 at 06:03 AM.
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