For AB, BC, CD, etc, the angles subtended at the centre are
AOB, BOC, COD, etc, and they are all equal because triangles AOB, BOC, etc are
congruent, (SSS) because two of the sides are equal to the radius of the circle
and the other is the side of the polygon. Each angle is therefore
radians, or
degrees ■.

The internal angles of the polygon are ABC, BCD, etc. The
angles ABO, OBC, etc are written as α, and the internal angles are 2α.

In each triangle, the angle at the centre is
radians, and as the triangles
OAB, OBC, etc are isosceles with OA, OB, etc as radii of the circle then 2α+=π,
being the angle sum of the triangle. Hence, 2α=radians
or degrees. Each internal angle is
2α, so each internal angle is
radians
or degrees ■.