f(x) = x^(3) / 3 - x^(2) / 3 - 2 * x
a = -2.2 (slider from -5 to 5, step 0.1)
A: (a, f(a))
b = Tangent(A, f)
f'(x) = f'(x)
texte1: "" + (f'(a)) + ""
texte2: "" + b + ""