# Find the maximum

• April 1st 2008, 04:55 AM
james_bond
Find the maximum
$f(x)=\frac {x-5}{x^2-5x+1992}$
For which $x$ does it have its maximum?
• April 1st 2008, 05:15 AM
mr fantastic
Quote:

Originally Posted by james_bond
$f(x)=\frac {x-5}{x^2-5x+1992}$
For which $x$ does it have its maximum?

Have you tried equating the derivative to zero and testing the nature of the resulting solutions?
• April 1st 2008, 05:47 AM
james_bond
No but unfortunately I need a solution without using calculus. Maybe with using some well-known inequalities (AM-GM, Cauchy, etc.) and some tricks...
• April 3rd 2008, 07:51 AM
JaneBennet
Quote:

Originally Posted by james_bond
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 doesn’t mean you can’t 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.
• April 5th 2008, 11:20 AM
james_bond
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 $x-5=\frac{1992}{x-5}\implies x=2\sqrt{498}+5$