Interior angle(s) of a polygon at its vertices
PolygonAngle[poly]
PolygonAngle[poly, p]
PolygonAngle[poly, i]
PolygonAngle[…, "spec"]
ℛ = Polygon[{{-1., 0.}, {0., 1.7320508075688772}, {1., 0.}}, {1, 3, 2}]; PolygonAngle[ℛ]
→ {1.0471975511965976, 1.0471975511965979, 1.0471975511965976}