求证明：
13Σx4≥16Σx3(y+z)≥13Σy2z2≥13Σx2yz\dfrac{1}{3}\Sigma x^4 \geq \dfrac{1}{6}\Sigma{x^3(y+z)} \geq \dfrac{1}{3}\Sigma y^2z^2 \geq \dfrac{1}{3}\Sigma x^2yz31Σx4≥61Σx3(y+z)≥31Σy2z2≥31Σx2yz
14Σa3≥112Σa2(b+c+d)≥14Σbcd\dfrac{1}{4}\Sigma a^3 \geq \dfrac{1}{12}\Sigma{a^2(b+c+d)} \geq \dfrac{1}{4} \Sigma bcd41Σa3≥121Σa2(b+c+d)≥41Σbcd
