Question 15
Let $ABC$ be a triangle with $AB = c$, $BC = a$ and $CA = b$. Show that
$$
\frac{\cos A}{a^3} + \frac{\cos B}{b^3} + \frac{\cos C}{c^3} \ge \frac{3}{2abc}.
$$
$$
\frac{\cos A}{a^3} + \frac{\cos B}{b^3} + \frac{\cos C}{c^3} \ge \frac{3}{2abc}.
$$