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Tuesday, June 7, 2016

Prove the following inequality holds: \sqrt{(\log_3a^b +\log_3a^c)}+\sqrt{(\log_3b^c +\log_3b^a)}+\sqrt{(\log_3c^a +\log_3c^b)}\le 3\sqrt{6}.

Let the reals a, b, c∈(1,\,∞) with a + b + c = 9.

Prove the following inequality holds:

\sqrt{(\log_3a^b +\log_3a^c)}+\sqrt{(\log_3b^c +\log_3b^a)}+\sqrt{(\log_3c^a +\log_3c^b)}\le 3\sqrt{6}.

My solution:

By applying the Cauchy-Schwarz inequality to the LHS of the intended inequality gives
\displaystyle \sqrt{(\log_3a^b +\log_3a^c)}+\sqrt{(\log_3b^c +\log_3b^a)}+\sqrt{(\log_3c^a +\log_3c^b)}

\displaystyle \le \sqrt{1+1+1}\sqrt{\log_3a^b +\log_3a^c+\log_3b^c +\log_3b^a+\log_3c^a +\log_3c^b}

\displaystyle =\sqrt{3}\sqrt{\log_3a^a +\log_3a^b+\log_3a^c+\log_3b^a +\log_3b^b+\log_3b^c+\log_3c^a +\log_3c^b+\log_3c^c-\log_3a^a-\log_3b^b-\log_3c^c}

\displaystyle =\sqrt{3}\sqrt{\log_3a^{a+b+c} +\log_3b^{a+b+c} +\log_3c^{a+b+c} -(\log_3a^a+\log_3b^b+\log_3c^c})

\displaystyle =\sqrt{3}\sqrt{(a+b+c)\log_3abc -(\log_3a^a+\log_3b^b+\log_3c^c})

\displaystyle =\sqrt{3}\sqrt{9\log_33^3 -(\log_3a^a+\log_3b^b+\log_3c^c}) since \displaystyle a+b+c=9\ge 3\sqrt[3]{abc}\implies abc \le 3^3

\displaystyle =\sqrt{3}\sqrt{27 -(a\log_3a+b\log_3b+c\log_3c})

Now, if we're to study the nature of the function for \displaystyle f(a)=a\log_3 a, we know it's a concave up and an increasing function, we could use the Jensen's inequality to figure out the minimum of \displaystyle a\log_3a+b\log_3b+c\log_3c:

\displaystyle \frac{a\log_3a+b\log_3b+c\log_3c}{3}\ge \frac{a+b+c}{3}\log_3\left(\frac{a+b+c}{3}\right)=\frac{9}{3}\log_3\left(\frac{9}{3}\right)=3

\displaystyle \therefore a\log_3a+b\log_3b+c\log_3c=3(3)=9

Now we get the maximum of the LHS of the intended inequality as:

\displaystyle \begin{align*}\sqrt{(\log_3a^b +\log_3a^c)}+\sqrt{(\log_3b^c +\log_3b^a)}+\sqrt{(\log_3c^a +\log_3c^b)}&\le \sqrt{3}\sqrt{27 -(a\log_3a+b\log_3b+c\log_3c})\\& \le \sqrt{3}\sqrt{27 -9}\\&=\sqrt{3}\sqrt{18}\\&=\sqrt{9}\sqrt{6}\\&=3\sqrt{6}\end{align*}

with equality when \displaystyle a=b=c=3.

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