Phép quay Q(O,π) là phép đối xứng tâm

α = 3.141592653589793 rad (slider from 0 to 6.283185307179586, step 0.017453292519943295)
O = (-2.8486656736488403, 5.735671185726327)
M = (4.071580917643194, 5.768008786620214)
a = Segment(O, M)
M' = Rotate(M, α, O)
b = Segment(O, M')
β = Angle(M, O, M')