Let $a,b,c>0$ satisfy $a\geq b\geq c$. Prove that $$\frac{4}{a^2(b+c)}+\frac{4}{b^2(c+a)}+\frac{4}{c^2(a+b)} \leq \left(\sum_{cyc} \frac{a^2+1} {b^2} \right)\left(\sum_{cyc} \frac{b^3}{a^2(a^3+2b^3)}\right).$$
Problem
Source: 2021 Thailand October Camp 1.2
Tags: inequalities
20.05.2023 09:18
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23.04.2024 13:06
Note that $\sum_{cyc}\frac{b^3}{a^2(a^3+2b^3)} = \frac{1}{2}\sum_{cyc}\frac{1}{a^2} - \sum_{cyc}\frac{a^3}{a^2(a^3+2b^3)}$ $\newline$ AM-GM : $$a^3 + 2b^3 \geqslant 3ab^2$$ Thus, $$ \sum_{cyc}\frac{b^3}{a^2(a^3+2b^3)} \geqslant \frac{1}{3}(\sum_{cyc}\frac{1}{a^2})$$AM-GM : $$\sum_{cyc}\frac{a^2+1}{b^2} \geqslant 2\sum_{cyc}\frac{a}{b^2}$$$\newline$ Cauchy schwarz : $$ 2\sum_{cyc}\frac{a}{b^2} \geqslant 2\sum_{cyc}\frac{1}{a} \geqslant \sum_{cyc}\frac{4}{a+b}$$Thus, $$\sum_{cyc}\frac{a^2+1}{b^2} \geqslant 4(\sum_{cyc}\frac{1}{a+b})$$Since $a\geqslant b \geqslant c \Rightarrow \frac{1}{b+c} \geqslant \frac{1}{c+a} \geqslant \frac{1}{a+b}, \frac{1}{a^2} \leqslant \frac{1}{b^2} \leqslant \frac{1}{c^2}$ Chebyshev : $$\frac{1}{3}( \frac{1}{b+c} + \frac{1}{c+a} + \frac{1}{a+b})( \frac{1}{a^2} + \frac{1}{b^2} + \frac{1}{c^2}) \geqslant \sum_{cyc}\frac{1}{a^2(b+c)}$$So, we get that $$\frac{4}{a^2(b+c)}+\frac{4}{b^2(c+a)}+\frac{4}{c^2(a+b)} \leq \left(\sum_{cyc} \frac{a^2+1} {b^2} \right)\left(\sum_{cyc} \frac{b^3}{a^2(a^3+2b^3)}\right).$$