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Let $a, b, c>1$. Show that: $$\frac{1}{1+\log_ab+log_bc}+\frac{1}{1+\log_bc+\log_ca}+\frac{1}{1+\log_ca+\log_ab}\leq1.$$

My attempt:

We noted $\log_bc=x, \log_ca=y, \log_ab=z$ with $xyz=1$ and we have reduced inequality at this time:$$2(x+y+z)\leq xy^2+x^2y+yz^2+y^2z+xz^2+x^2z.$$ Does anyone have idea how it goes? Thank You!

I found a simple continuation: $$2(x+y+z)\leq xy^2+x^2y+yz^2+y^2z+xz^2+x^2z\iff$$ $$\iff2(x+y+z)\leq xy(x+y)+yz(y+z)+zx(z+x)\iff$$ $$\iff2(x+y+z)\leq (xy+yz+zx)(x+y+z)-3xyz\iff$$ $$\iff3\leq(x+y+z)(xy+yz+zx-2).$$ The last inequality follows easily using: $$3\leq x+y+z$$ $$1=3(xyz)^{\frac{2}{3}} -2\leq xy+yz+zx-2.$$ These last inequality is obtained by applying AM-GM inequality.

medicu
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2 Answers2

5

other solution:

since $xyz=1$,we only prove $$\dfrac{1}{1+x+y}+\dfrac{1}{1+y+z}+\dfrac{1}{1+x+z}\le 1$$ let $$x=\dfrac{a^2}{bc},y=\dfrac{b^2}{ac},z=\dfrac{c^2}{ab}$$ so $$\Longrightarrow \dfrac{1}{1+y+z}=\dfrac{1}{1+\dfrac{b^2}{ac}+\dfrac{c^2}{ab}}=\dfrac{abc}{abc+b^3+c^3}$$ so this inequality we only prove $$\Longleftrightarrow \dfrac{abc}{abc+a^3+c^3}+\dfrac{abc}{abc+b^3+c^3}+\dfrac{abc}{abc+a^3+b^3}\le 1$$ note $$a^3+b^3\ge ab(a+b)\Longrightarrow a^3+b^3+abc\ge ab(a+b+c)$$ $$\Longrightarrow \dfrac{abc}{a^3+b^3+abc}\le\dfrac{abc}{ab(a+b+c)}=\dfrac{c}{a+b+c}$$ so $$\sum_{cyc}\dfrac{abc}{a^3+b^3+abc}\le\sum_{cyc}\dfrac{c}{a+b+c}=1$$

math110
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Following OP's simplification of the inequality:

$\displaystyle \begin{align} xy^2+x^2y+yz^2+y^2z+xz^2+x^2z &= \sum\limits_{cyc} (x^2y + xy^2) \\ &=\sum\limits_{cyc} (x^{6/3}y^{3/3} + x^{3/3}y^{6/3}) \\ & \ge \sum\limits_{cyc} (x^{5/3}y^{4/3} + x^{4/3}y^{5/3}) \tag{1}\\ &=\sum\limits_{cyc} x^{5/3}(y^{4/3} + z^{4/3})\tag{2} \\ & \ge 2\sum\limits_{cyc} x^{5/3}y^{2/3}z^{2/3} \tag{3}\\ &= 2\sum\limits_{cyc} x \end{align}$

Justification of steps:

$(1) \, (x^{6/3}y^{3/3} + x^{3/3}y^{6/3}) - (x^{5/3}y^{4/3} + x^{4/3}y^{5/3}) = xy(x^{1/3}+y^{1/3})(x^{1/3}-y^{1/3})^2 \ge 0$

$(2)$ Rearranging the cyclic summation.

$(3) \, y^{4/3} + z^{4/3} \ge 2y^{2/3}z^{2/3}$ and put $xyz = 1$.

r9m
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