f(x) = 1 / (1 + x^(2))
A = Point(f)
a = Tangent(A, f)
d: y = 1 / (1 + x(A)^(2)) - 2 * x(A) * (x - x(A)) / (1 + x(A)^(2))^(2) + (3 * x(A)^(2) - 1) * (x - x(A))^(2) / (1 + x(A)^(2))^(3)