[The jumpers]

ใช้ Holder inequality มั้ง!
\[\sum_{cyc}a^{\frac{1}{3}}(a^2+2ab)^{\frac{1}{3}}\leqslant 3^{\frac{1}{3}}(a+b+c)^{\frac{1}{3}}(\sum_{cyc}(a^2+2bc))^{\frac{1}{3}}=\sqrt[3]{3}(a+b+c)\]