Maths Olympiad Prep

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Algebra Difficulty 7.2 National olympiad, round 2 Prove it

Example 12 Let a,b,ca, b, c be positive real numbers, and satisfy abc=1abc=1. Try to prove: 1a3(b+c)+1b3(c+a)+\frac{1}{a^{3}(b+c)}+\frac{1}{b^{3}(c+a)}+ 1c3(a+b)32\frac{1}{c^{3}(a+b)} \geqslant \frac{3}{2}. (36th IMO Problem)

Solution

To prove the inequality, we homogenize both ends, which is equivalent to proving:
1a3(bc)+1b3(c+a)+1c3(a+b)32(abc)43\frac{1}{a^{3}(b c)}+\frac{1}{b^{3}(c+a)}+\frac{1}{c^{3}(a+b)} \geqslant \frac{3}{2(a b c)^{\frac{4}{3}}}

Let a=x3,b=y3,c=z3a=x^{3}, b=y^{3}, c=z^{3}, substituting into the above inequality, we get:
cyc1x9(y3+z3)32x4y4z4\sum_{c y c} \frac{1}{x^{9}\left(y^{3}+z^{3}\right)} \geqslant \frac{3}{2 x^{4} y^{4} z^{4}}

By Muirhead's inequality, we have:

That is,
(symx12y12symx11y8z5)+2(symx12y9z3symx11y8z5)+(symx9y9z6symx8y8z8)0\begin{array}{l} \left(\sum_{s y m} x^{12} y^{12}-\sum_{s y m} x^{11} y^{8} z^{5}\right)+2\left(\sum_{s y m} x^{12} y^{9} z^{3}-\sum_{s y m} x^{11} y^{8} z^{5}\right)+ \\ \left(\sum_{s y m} x^{9} y^{9} z^{6}-\sum_{s y m} x^{8} y^{8} z^{8}\right) \geqslant 0 \end{array}
symx12y12+2symx12y9z3+symx9y9z63symx11y8z5+6x8y8z8\sum_{s y m} x^{12} y^{12}+2 \sum_{s y m} x^{12} y^{9} z^{3}+\sum_{s y m} x^{9} y^{9} z^{6} \geqslant 3 \sum_{s y m} x^{11} y^{8} z^{5}+6 x^{8} y^{8} z^{8}

Therefore, inequality (1) holds, and thus the original inequality holds.

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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.