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Algebra Difficulty 6.9 National olympiad Prove it

Example 17 Given that a,b,ca, b, c are all positive numbers, and satisfy abc=1abc=1.
Prove: 1a3(b+c)+1b3(c+a)+1c3(a+b)32\frac{1}{a^{3}(b+c)}+\frac{1}{b^{3}(c+a)}+\frac{1}{c^{3}(a+b)} \geqslant \frac{3}{2}.

Solution

Analysis: The original inequality is equivalent to b2c2a(b+c)+c2a2b(c+a)\frac{b^{2} c^{2}}{a(b+c)}+\frac{c^{2} a^{2}}{b(c+a)} +a2b2c(a+b)32+\frac{a^{2} b^{2}}{c(a+b)} \geqslant \frac{3}{2}. When a=b=c=1a=b=c=1, equality holds, at which point b2c2a(b+c)=a(b+c)4,c2a2b(c+a)=b(c+a)4,a2b2c(a+b)\frac{b^{2} c^{2}}{a(b+c)}=\frac{a(b+c)}{4}, \frac{c^{2} a^{2}}{b(c+a)}=\frac{b(c+a)}{4}, \frac{a^{2} b^{2}}{c(a+b)} =c(a+b)4=\frac{c(a+b)}{4}, and b2c2a(b+c)+a(b+c)4bc,c2a2b(c+a)+\frac{b^{2} c^{2}}{a(b+c)}+\frac{a(b+c)}{4} \geqslant b c, \frac{c^{2} a^{2}}{b(c+a)}+ b(c+a)4ca,a2b2c(a+b)+c(a+b)4ab\frac{b(c+a)}{4} \geqslant c a, \frac{a^{2} b^{2}}{c(a+b)}+\frac{c(a+b)}{4} \geqslant a b. Adding the last three inequalities side by side yields b2c2a(b+c)+c2a2b(c+a)+a2b2c(a+b)12(ab\frac{b^{2} c^{2}}{a(b+c)}+\frac{c^{2} a^{2}}{b(c+a)}+\frac{a^{2} b^{2}}{c(a+b)} \geqslant \frac{1}{2}(a b +bc+ca)32(abc)23=32+b c+c a) \geqslant \frac{3}{2} \sqrt[3]{(a b c)^{2}}=\frac{3}{2}.

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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.