f(x) = (-0.0004 * x^(3)) + (0.018 * x^(2)) + 0.8
a: f(15)
f0(x) = If[0 ≤ x ≤ 20, f(x)]
p(x) = If[20 ≤ x ≤ 40, f(x)]
f1(x) = If[0 ≤ x ≤ 40, f'(x)]
P = Point(f)
gl1: x = x(P)