1 rad

O = (-1.48, 4.08)
r_1 = 1.5 (slider from 0.3 to 3, step 0.1)
c = Circle(O, r_1)
A = Point(c)
B = Rotate(A, 57.29577951308232*°, O)
α = Angle(A, O, B)
r = Segment(A, O)
b = Segment(O, B)
a = CircleArc(O, A, B)