S_1 = (10.12, -3.6)
a = 0 (slider from 0 to 2, step 1)
S_2 = (18.52, -4.36)
T = Midpoint(S_1, S_2)
k_1 = Circle(S_1, T)
k_2 = Circle(S_2, T)
f = Line(S_1, S_2)
g = OrthogonalLine(T, f)