A inequality proposed at Zhautykov Olympiad 2008
Let be $a,b,c >0$ with $abc=1$. Prove that:
$$\sum_{cyc}{\frac{1}{(a+b)b}} \geq \frac{3}{2}.$$
$a=\frac{x}{y}$, $b=\frac{y}{z}$, $c=\frac{z}{x}$.
Our inequality becomes: $$\sum_{cyc}{\frac{z^2}{zx+y^2}} \geq \frac{3}{2}.$$ Now we use that: $z^2+x^2 \geq 2zx.$ $$\sum_{cyc}{\frac{z^2}{zx+y^2}} \geq \sum_{cyc}{\frac{2z^2}{z^2+x^2+2y^2}} \geq \frac{3}{2}.$$
Now applying Cauchy Schwarz we obtain the desired result .
What I wrote can be found on this link: mateforum. But now, I don't know how to apply Cauchy-Schwarz .
Thanks:)