Let $\vec F = (3 a^2 x + 10 a y^2)\vec i + (10 z^3 - 2 a y)\vec j - ( z + 10 x^2 + 10 y^2)\vec k$.

(a) Find the value(s) of $a$ making $\mbox{div }\vec F = 0$
$a =$
(Enter your value, or if you have more than one, enter a comma-separated list of your values.)

(b) Find the value(s) of $a$ making $\mbox{div }\vec F$ a minimum
$a =$
(Enter your value, or if you have more than one, enter a comma-separated list of your values.)