so the stationary points correspond to the roots of:
1.8r - 3 r^2=0,
which are r=0, and r=0.6. Now d^2S/dr^2 is positive at r=0, so this is
a local minimum, and d^2S/dr^2 is negative at r=0.6 so this is a local
maximum, and S(0.6)=1.108 F.
This is also a global maximum, as r must be greater than 0.