\overrightarrow{ST} · \overrightarrow{XT}=0

S = (2.1751572669595816, 2.3608457099696682)
r = 2 (slider from 1 to 4, step 1)
k = Circle(S, r)
X = (5.316867885126089, 1.5221687229167415)
t, g = Tangent(X, k)
T = Intersect(k, t)
u = Vector(S, T)
v = Vector(X, T)