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)
x0: x(P)
Q: (x0, f1(x0))
gL = -5 (slider from -5 to 5, step 0.1)
gR = 40 (slider from -5 to 50, step 0.1)