$\displaystyle v= {10 \pi r^2(6-r) /3 $
the whole is meant to be over 3
ok i really gotta practice with pi in these eqn's.
$\displaystyle v = 20\pi r^2 - \frac{10\pi}{3} r^3 $
$\displaystyle v' = 40 \pi r - 10 \pi r^2 $ for stat points v' = 0
$\displaystyle 10 \pi r (4 - r) = 0 $
therefore
$\displaystyle r= 0 or 4 $
0 doesnt make sense so 4 only?