Maths Olympiad Prep

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Algebra Difficulty 6.3 National olympiad Prove it

110. Let a,b,ca, b, c be real numbers greater than 1. Prove:
logabc+logbca+logcab4(logabc+logbca+logcab)\log _{a} b c+\log _{b} c a+\log _{c} a b \geqslant 4\left(\log _{a b} c+\log _{b c} a+\log _{c a} b\right)
(2008 Croatian Mathematical Olympiad, 2010 Malaysian Mathematical Olympiad)

Solution

110. Let x=lga,y=lgb,z=lgcx=\lg a, y=\lg b, z=\lg c, since a,b,ca, b, c are real numbers greater than 1, so x,y,z>x, y, z > 0.
logabc+logbca+logcab4(logabc+logbca+logcab)xy+xz+yz+yx+zx+zy4xy+z+4yz+x+4zx+y\begin{array}{l} \log _{a} b c+\log _{b} c a+\log _{c} a b \geqslant 4\left(\log _{a b} c+\log _{b c} a+\log _{c a} b\right) \Leftrightarrow \\ \frac{x}{y}+\frac{x}{z}+\frac{y}{z}+\frac{y}{x}+\frac{z}{x}+\frac{z}{y} \geqslant \frac{4 x}{y+z}+\frac{4 y}{z+x}+\frac{4 z}{x+y} \end{array}

It is only necessary to prove zx+zy4zx+y\frac{z}{x}+\frac{z}{y} \geqslant \frac{4 z}{x+y}, etc., which is equivalent to proving 1x+1y4x+y\frac{1}{x}+\frac{1}{y} \geqslant \frac{4}{x+y}. This is obvious.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.