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Wednesday, April 6, 2016

Let a,b and c be positive real numbers with abc=1, prove that a2+bc+b2+ca+c2+ab1

Let a,b and c be positive real numbers with abc=1, prove that

a2+bc+b2+ca+c2+ab1

In the problem 4 as shown in quiz 22, I asked if you could approach the problem using the Hölder's inequality, I hope you have tried it before checking out with my solution to see why the Hölder's inequality wouldn't help:

The following inequality is always true (by the Hölder's inequality) for all positive a, b and c such that abc=1:

(cyclic((a13)3((2+bc)13)3))13(cyclic((2+bc)13)3))13(cyclic(1((a)13)3))13

cyclic(a13(2+bc)13(2+bc)131(a)13)

\displaystyle = \sum_{\text{cyclic}}\left(\frac{\cancel{a^{\frac{1}{3}}}}{\cancel{(2+bc)^{\frac{1}{3}}}}\cdot \cancel{(2+bc)^{\frac{1}{3}}} \cdot \frac{1}{\cancel{(a)^{\frac{1}{3}}}}\right)

\displaystyle = \sum_{\text{cyclic}}\left(1\right)

\displaystyle = 3

Therefore by cubing both sides of the inequality we see that we have:

\displaystyle \left(\sum_{\text{cyclic}}\left(\frac{(a^{\frac{1}{3}})^3}{(2+bc)^{\frac{1}{3}})^3}\right)\right)\left(\sum_{\text{cyclic}}\left((2+bc)^{\frac{1}{3}})^3\right)\right)\left(\sum_{\text{cyclic}}\left(\frac{1}{((a)^{\frac{1}{3}})^3}\right)\right)\ge 3^3

\displaystyle \left(\sum_{\text{cyclic}}\left(\frac{a}{2+bc}\right)\right)\left(\sum_{\text{cyclic}}\left(2+bc\right)\right)\left(\sum_{\text{cyclic}}\left(\frac{1}{a}\right)\right)\ge 3^3

which, upon simplification, get down to

\displaystyle \color{yellow}\bbox[5px,purple]{\left(\sum_{\text{cyclic}}\left(\frac{a}{2+bc}\right)\right)\left(6+ab+bc+ca\right)\left(ab+bc+ca\right)\ge 3^3}

which also means, if we can prove

\displaystyle \color{yellow}\bbox[5px,green]{\left(\sum_{\text{cyclic}}\left(\frac{a}{2+bc}\right)\right) \ge \frac{3^3}{\left(6+ab+bc+ca\right)\left(ab+bc+ca\right)}}=1

then we're done.

But note that we haven't enough data to look for the maximum for \displaystyle ab+bc+ca, this suggests this method has no exit, no way out.




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