01  #include<stdio.h>
02  #include<math.h>
03  
04  int main()
05  {
06   int side;
07   float area;
08  
09   printf("nEnter the Length of the Side : ");
10   scanf("%d", &side);
11  
12   area = 2*(1+sqrt(2))*side*side;
13  
14   printf("nArea of Regular Octagon : %.2f", area);
15   return 0;
16  }

 
 OUTPUT :
 
 Enter the Length of Side : 3
 Area of Regular Octagon : 43.46