Let a,b and c be positive real numbers with abc=1, prove that
a2+bc+b2+ca+c2+ab≥1
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)13⋅1(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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