Octagon eight lines of reflective symmetry and rotational symmetry of order 8. A regular octagon is represented by the Schläfli symbol {8}. The internal angle at each vertex of a regular octagon is 135° (formula_1 radians). The central angle is 45° (formula_2 radians). The area of a regular octagon of side length "a" is given by In terms of the circumradius "R", the area is In terms of the apothem "r" (see also inscribed figure), the area is These last two coefficients bracket the value of pi, the area of the unit circle. The area can also be expressed as where
Angle "AOC", then The measure of the angle "AOC" is the sum of the measure of angle AOB and the measure of angle "BOC". In this postulate it does not matter in which unit the angle is measured as long as each angle is measured in the same unit. Units used to represent angles are listed below in descending magnitude order. Of these units, the "degree" and the "radian" are by far the most commonly used. Angles expressed in radians are dimensionless for the purposes of dimensional analysis. Most units of angular measurement are defined such that one "turn" (i.e. one