\frac {a}{b+2c}+\frac {b}{c+2a}+\frac {c}{a+2b}=\frac {a^2}{ab+2ac}+\frac {b^2}{bc+2ab}+\frac {c^2}{ac+2bc}\ge \frac{(a+b+c)^2}{3(ab+bc+ac)}