a = 4 (slider from -10 to 10, step 0.1)
f_a(x) = (x - 2) / ((10 * (a - x)))
P = Point(f_a)
Tangente = Tangent(P, f_a)
Normale = OrthogonalLine(P, Tangente)
f_a' = Derivative(f_a)