llaunes y contingut y preu

llauna <==> 2pi·rh

contingut <==> pi·r^{2}·h

(f(r)/pi) = p·2rh+(-n)·r^{2}·h

d_{r}[ (f(r)/pi) ] = p·2h+(-n)·2rh = 0

p = n·r