Rechteck

a = 1 (slider from -5 to 5, step 0.1)
f(x) = (1 / 6 * x^(3)) - ((1 / 2 * a^(2)) * x) + a
A, B = Extremum(f)
g(x, k) = (1 / 6 * x^(3)) - ((1 / 2 * k^(2)) * x) + k
C_{1}: (x(A), y(B))
D: (x(B), y(A))
r, a_1, c, b, d = Polygon(A, C_{1}, B, D)
C = TurningPoint(f)
e = true
Text1: "f_{" + a + "}"