r = 1.5 (slider from 0.2 to 5, step 0.1)
O = (-1.7, 4.1)
c = Circle(O, r)
B = Point(c)
A = Point(c)
α = Angle(A, O, B)
a_1 = Segment(O, A)
b = Segment(O, B)
C = Midpoint(A, B)
d = Ray(O, C)
M = Intersect(c, d, 1)
a = CircleArc(O, A, B)
β: α * π / 180
γ: β * r