For $\displaystyle x,y,z>0$, prove

$\displaystyle x^{5}+y^{5}+z^{5}\leq x^{5}\sqrt{\frac{x^{2}}{yz}}+y^{5}\sqrt{\frac{y^{2 }}{xz}}+z^{5}\sqrt{\frac{z^{2}}{xy}}.$

thanks.

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- Nov 10th 2008, 11:44 PMbkarpuzAn algebric inequality
For $\displaystyle x,y,z>0$, prove

$\displaystyle x^{5}+y^{5}+z^{5}\leq x^{5}\sqrt{\frac{x^{2}}{yz}}+y^{5}\sqrt{\frac{y^{2 }}{xz}}+z^{5}\sqrt{\frac{z^{2}}{xy}}.$

thanks. - Nov 11th 2008, 12:07 AMNonCommAlg
- Nov 11th 2008, 12:09 AMbkarpuz
- Nov 11th 2008, 12:52 AMNonCommAlg
because of symmetry, we may assume that $\displaystyle x \leq y \leq z.$ then obviously: $\displaystyle \sqrt{\frac{x^{2}}{yz}} \leq \sqrt{\frac{y^{2}}{xz}} \leq \sqrt{\frac{z^{2}}{xy}}.$ hence by Chebyshev's inequality we'll have:

$\displaystyle x^{5}\sqrt{\frac{x^{2}}{yz}}+y^{5}\sqrt{\frac{y^{2 }}{xz}}+z^{5}\sqrt{\frac{z^{2}}{xy}} \geq \frac{1}{3}(x^5 + y^5 + z^5) \left(\sqrt{\frac{x^{2}}{yz}}+\sqrt{\frac{y^{2}}{x z}}+\sqrt{\frac{z^{2}}{xy}} \right).$ but by AM-GM we have: $\displaystyle \sqrt{\frac{x^{2}}{yz}}+\sqrt{\frac{y^{2}}{xz}}+\s qrt{\frac{z^{2}}{xy}} \geq 3. \ \ \ \ \ \Box$

imagine how many inequalities you can create using only Chebyshev's inequality + AM-GM inequality! i guess i'm back to high school again! (Mmm) - Nov 11th 2008, 01:03 AMbkarpuz
I was thinking of a different solution.

I know that this inequality can be shown by special inequalities, the first one dropped in my mind was Muirhead's inequality (again because of the symmetry), which you may find in Muirhead's inequality - Wikipedia, the free encyclopedia.

If there is any other solutions, I would like to see.

Thanks**NonCommAlg**. - Nov 11th 2008, 01:32 AMNonCommAlg
when i was a high school kid, we were trained to avoid using Muirhead's inequality as much as we can! i never knew why? i guess i'm still under the infulence! lol

anyway, yes, Muirhead's inequality will also solve the problem because if you multiply both sides of the inequality by $\displaystyle xyz$ you'll get: $\displaystyle \sum_{sym}x^6yz \leq \sum_{sym}x^7 \sqrt{y}\sqrt{z},$

which is true because $\displaystyle 7, \frac{1}{2}, \frac{1}{2}$ majorizes $\displaystyle 6, 1, 1.$