K(x) = a·x^3 + b·x^2 + c·x + d
k(x) = a·x^2 + b·x + c + d/x
kv(x) = a·x^2 + b·x + c
kv'(x) = 2·a·x + b
K(0) = 0 --> d = 30000
K(20) = 60000 --> a·20^3 + b·20^2 + c·20 + d = 60000
k(60) = 1200 --> a·60^2 + b·60 + c + d/60 = 1200
kv'(60) = 0 --> 2·a·60 + b = 0