How do we find the area of a regular polygon?

*We find the are of a regular polygon by the formula 1/2nas or 1/2 P.A , where A is the area and P is the perimeter , a is the apothegm , s is the length  of each side, and n is the number of sides. In other words the Area=nas/2 

for example
 - the number of sides are 5
-the apothegm is 2 cm
- and the the length is 8 cm

 so in this case we multiply 5*8*2 =  8 and the we divided it by 2
which give us 40cm²