A: (0, 0)
B = Point(Circle[A, 10])
f = Segment(A, B)
a = 90 (slider from 0 to 180)
C = Rotate(B, a°, A)
α = Angle(B, A, C)
t1, c, a_1, b = Polygon(A, B, C)
D: (a, t1)