Difference between revisions of "1974 USAMO Problems/Problem 2"
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*[http://www.mathlinks.ro/viewtopic.php?t=98846 Hard inequality] | *[http://www.mathlinks.ro/viewtopic.php?t=98846 Hard inequality] | ||
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*[http://www.mathlinks.ro/viewtopic.php?t=213258 ineq] | *[http://www.mathlinks.ro/viewtopic.php?t=213258 ineq] | ||
*[http://www.mathlinks.ro/Forum/viewtopic.php?t=46247 exponents (generalization)] | *[http://www.mathlinks.ro/Forum/viewtopic.php?t=46247 exponents (generalization)] | ||
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[[Category:Olympiad Algebra Problems]] | [[Category:Olympiad Algebra Problems]] | ||
[[Category:Olympiad Inequality Problems]] | [[Category:Olympiad Inequality Problems]] |
Revision as of 14:56, 17 September 2012
Problem
Prove that if ,
, and
are positive real numbers, then
![$a^ab^bc^c\ge (abc)^{(a+b+c)/3}$](http://latex.artofproblemsolving.com/c/d/b/cdb94a39e3190f57fcbb1f2fbe849a6c038b66e7.png)
Solution
Consider the function .
for
; therefore, it is a convex function and we can apply Jensen's Inequality:
![$\frac{a\ln{a}+b\ln{b}+c\ln{c}}{3}\ge \left(\frac{a+b+c}{3}\right)\ln\left(\frac{a+b+c}{3}\right)$](http://latex.artofproblemsolving.com/d/b/0/db0d87edcf1329bb08f89821fd547f050f23f363.png)
Apply AM-GM to get
![$\frac{a+b+c}{3}\ge \sqrt[3]{abc}$](http://latex.artofproblemsolving.com/8/7/2/872ec383ca01e8997916c1dbaee116fee66f8130.png)
which implies
![$\frac{a\ln{a}+b\ln{b}+c\ln{c}}{3}\ge \left(\frac{a+b+c}{3}\right)\ln\left(\sqrt[3]{abc}\right)$](http://latex.artofproblemsolving.com/d/a/3/da34b15dd7c8d75509c763b00ecc8f379c8cbd22.png)
Rearranging,
![$a\ln{a}+b\ln{b}+c\ln{c}\ge\left(\frac{a+b+c}{3}\right)\ln\left(abc\right)$](http://latex.artofproblemsolving.com/3/f/5/3f59079c18ed4396607cf01df15d21c4dcb3d272.png)
Because is an increasing function, we can conclude that:
![$e^{a\ln{a}+b\ln{b}+c\ln{c}}\ge{e}^{\ln\left(abc\right)(a+b+c)/3}$](http://latex.artofproblemsolving.com/6/3/5/635872e58e16d76df675269c7674dff286ad4dd5.png)
which simplifies to the desired inequality.
Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.
See Also
1974 USAMO (Problems • Resources) | ||
Preceded by Problem 1 |
Followed by Problem 3 | |
1 • 2 • 3 • 4 • 5 | ||
All USAMO Problems and Solutions |