C = Function(0.5 * x^(3.0) - 20.0 * x^(2.0) + 300.0 * x + 1200.0, 30.0, 40.0)
a = 34 (slider from 30 to 40, step 1)
M: (a, C(a))
N: (a + 1.0, C(a + 1.0))
b = Line(M, N)
c = Tangent(M, C)