Cho a,b,c CMR :a) a/b ≥ a^2 + ab - b^2

Cho a,b,c > 0. CMR :$$\begin{gathered} a)\frac{a^3}{b}\ge a^2+ab-b^2 \\ b) \frac{a^3}{b}+\frac{b^3}{c}+\frac{c^3}{a}\ge ab+bc+ca \end{gathered}$$