Lien entre f et f'

f(x) = (1 / 3 * x^(3)) - (1 / 2 * x^(2)) - (2 * x) + 1 / 3
a = 0.4 (slider from -5 to 5, step 0.1)
A: (a, f(a))
g = Tangent(A, f)
a_1 = Slope(g)