t = -1.2 (slider from -20 to 20, step 0.1)
h = 0.49 (slider from 0.01 to 2, step 0.01)
f(x) = 2.0 * x / (1.0 + x^(2.0))
g(x) = (1.0 - x^(2.0)) / (1.0 + x^(2.0))
A: (f(t), g(t))
B: (f(t + h), g(t + h))
O = Intersect(xAxis, yAxis)
u = Vector(O, A)
v = Vector(O, B)
w = Vector(A, B)
C: A + w / h
z = Vector(A, C)
D: A + (f'(t), g'(t))
a = Vector(A, D)