Cho a,b,c>0. CMR \frac{b^2}{a(a^2+b^2)}+\frac{b^2}{b(b^2+c^2)}+\frac{a^2}{c(c^2+a^2)}\geq \frac{1}{2}(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})