In: Advanced Math
For p, q ∈ S^1, the unit circle in the plane, let
d_a(p, q) = min{|angle(p) − angle(q)| , 2π − |angle(p) −
angle(q)|}
where angle(z) ∈ [0, 2π) refers to the angle that z makes with the
positive x-axis.
Use your geometric talent to prove that d_a is a metric on S^1.