Length of a circumference of a circle.

Here you have to move the slider point

α = 3.7524578917878086 rad (slider from 0 to 6.28318530717959, step 0.0174532925199433)
A = (18.016014647661734, -3.437997884675339)
B = (18, -8)
c = Circle(A, B)
B' = Rotate(B, -α, A)
β = Angle(B', A, B)
a = Segment(B', A)
b = Segment(B, A)
d = CircleArc(A, B', B)
C = Point(Circle[B, d])
e = Segment(B, C)