Cho $x,y,z>0$ sao cho $xy+yz+zx=1$ . Chứng minh rằng:
$\frac{2x}{1+{{x}^{2}}}+\frac{2y}{1+{{y}^{2}}}+\frac{2z}{1+{{z}^{2}}}\le \frac{1}{\sqrt{1+{{x}^{2}}}}+\frac{1}{\sqrt{1+{{y}^{2}}}}+\frac{1}{\sqrt{1+{{z}^{2}}}}$
Edited by santo3vong, 23-09-2015 - 15:44.