A polygon with all sides having equal length and all angles are equal is called as regular polygon. The line segment from the center to the midpoint of any of the sides is called as the apothem of regular polygon. There are two different formulas to find the apothem.

r = a / [2 tan(π / n)]

[or]

r = R cos (π / n)

R = Circumradius

π = 3.14159

a = Side length

n = Number of side

To find the apothem of a regular polygon, either you want to know the side length or the circumradius. Length of the side can also be calculated from the perimeter. But both formulas can be used only if number of sides is known.