Cho x + y + z = 0 và xyz khác 0. CMR: x^3 + y^3 + z^3

$$\begin{gathered} \mathrm{Cho} \mathrm{x}+\mathrm{y}+\mathrm{z}=0 \\ \mathrm{xyz} \mathrm{khác} 0 \\ \mathrm{a} \mathrm{CMR} : {\mathrm{x}}^3+{\mathrm{y}}^3+{\mathrm{z}}^3 \\ \mathrm{bTính} \mathrm{D}= \frac{{\mathrm{x}}^2}{\mathrm{yz}}+\frac{{\mathrm{y}}^2}{\mathrm{xz}}+\frac{{\mathrm{z}}^2}{\mathrm{xy}}. \end{gathered}$$