I HAVE THE SAME PROBLEM PLEASE HELP US....

My program is :

#include<iostream>
#include<cmath>
#define PI 3.14159265358979323846264338327950288
using namespace std;
double r,a,whole,reminder,side,leftSide;
int main()
{
   cin>>r>>a;
   whole=180/a;
   reminder=360-whole*(2.0*a);
   side=2*r*sin(a*PI/180);
   leftSide=2*r*sin(reminder*PI/360.0);
   printf("%.12lf",r+(whole-1)*side+leftSide);
   system("pause");
   return 0;
}

you forget about last part

for ex input 1 40

you have 3 part with arch = 80 degrees

and last 40 degrees (not 80)

SOrry for my english
I hope that you understand

Test 5:
Input 10.000 30.001
Output 59.999698