t = 2.8 (slider from -5 to 5, step 0.1)
A: (t, t * (4.0 - t))
f(x) = x * (4.0 - x)
O = Point(f)
u = Vector(O, A)
h = 0.1 (slider from 0.01 to 1, step 0.01)
B: (t + h, f(t + h))
v = Vector(O, B)
w = Vector(A, B)
C: A + w / h
z = Vector(A, C)