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Prove $$\frac{\sqrt[3]{ab}+\sqrt[3]{ac}}{2\left(\sqrt[3]{ab}+\sqrt[3]{bc}+\sqrt[3]{ac}\right)}\cdot\left(\sqrt[3]{abc}+5\right) +4a\ge 2\sqrt{4a^2+5a}, \tag{*}$$when $a,b,c>0.$

In the comment section of this topic, River Li write about an isolated fudging as an shortly quick way for a non-homogenous inequality $$4(a+b+c)+\sqrt[3]{abc}+5\ge 2\left(\sqrt{4a^2+5a}+\sqrt{4b^2+5b}+\sqrt{4c^2+5c}\right). $$

I was quite curious about how the solver come up with $(*).$

Also, I tried to prove $(*)$ by squaring but it's very complicated.

Hope you share some thoughts for this isolated fudging. Thanks for interest.

Relevant information.

There're some non-homogenous inequality posted in Mathematics Stack Exchange. I leave it here.

Problem 1, Problem 2, Problem 3, Problem 4.

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1 Answers1

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Proof.

We'll prove the following general result for $a,b,c>0,$ the constant $0\le k\le 5$ and an arbitrary negative expression $T$ represent to $a,b,c.$

$$\color{black}{\frac{\sqrt[3]{ab}+\sqrt[3]{ac}}{2\left(\sqrt[3]{ab}+\sqrt[3]{bc}+\sqrt[3]{ac}\right)}\cdot\left((k+1)\sqrt[3]{abc}+(5-k)T\right) +4a\ge 2\sqrt{a\left(4a+k\sqrt[3]{abc}+(5-k)T\right) }.} \tag{*}$$

For easily verifying, we prove $$\color{black}{\frac{ab+ac}{2\left(ab+bc+ca\right)}\cdot\left((k+1)abc+(5-k)T\right) +4a^3\ge 2\sqrt{a^3\left(4a^3+kabc+(5-k)T\right) }.}$$ or $$\color{black}{\frac{b+c}{2\left(ab+bc+ca\right)}\cdot\left[(k+1)bc+\dfrac{(5-k)T}{a}\right] +4a\ge 2\sqrt{4a^2+kbc+\dfrac{(5-k)T}{a} }.} \tag{1}$$

Notice that by AM-GM $$\frac{b+c}{2\left(ab+bc+ca\right)}=\frac{1}{2a+\dfrac{2bc}{b+c}}\ge \frac{1}{2a+\sqrt{bc}}.$$Thus, it's enough to prove the stronger inequality of $(1)$ $$\frac{(k+1)bc+\dfrac{(5-k)T}{a}}{2a+\sqrt{bc}}+4a\ge 2\sqrt{4a^2+kbc+\dfrac{(5-k)T}{a} },$$which is true by AM-GM.

Indeed, \begin{align*} \frac{(k+1)bc+\dfrac{(5-k)T}{a}}{2a+\sqrt{bc}}+4a&=\frac{(k+1)bc+\dfrac{(5-k)T}{a}}{2a+\sqrt{bc}}+2a-\sqrt{bc}+2a+\sqrt{bc}\\&= \frac{4a^2+kbc+\dfrac{(5-k)T}{a}}{2a+\sqrt{bc}}+2a+\sqrt{bc}\\&\ge 2\sqrt{4a^2+kbc+\dfrac{(5-k)T}{a} }. \end{align*}

Hence, $(1)$ is proven and $(*)$ is subsequently true.

When $k=0; T=1$ we obtain $$\color{black}{\frac{\sqrt[3]{ab}+\sqrt[3]{ac}}{2\left(\sqrt[3]{ab}+\sqrt[3]{bc}+\sqrt[3]{ac}\right)}\cdot\left(\sqrt[3]{abc}+5\right) +4a\ge 2\sqrt{a\left(4a+5\right) }.}$$

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