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