Lightoj 1433: Minimum Arc Distance

Problem link:[http://lightoj.com/volume_showproblem.php?problem=1433]
Catagory: Adhoc, Mathematics

Explanation:
          S=r*s
here,
          S= Arc length between 2 points in circle
          r= radius of circle
          s= Angle between 2 points in circle
again,
         c^2=a^2 + b ^2 - 2abcos(angle)
         here, a=b=r
         and c=distace between 2 points

so, angle=Inverse_cos((2r^2-c^2)/r^2)

Code:

#include<bits/stdc++.h>
#define ll       long long int
#define SIZE 100005
using namespace std;

double dis(ll x1, ll x2, ll y1, ll y2)
{
          double m=(x1-x2)*(x1-x2);
          double n=(y1-y2)*(y1-y2);

          return sqrt(m+n);

}


int main()
{
          ll o1,o2,a1,a2,b1,b2,l;
          cin>>l;
          ll i=1;
          while(i<=l)
          {
                    cin>>o1>>o2>>a1>>a2>>b1>>b2;
                    double r=dis(o1,a1,o2,a2);
                    double d=dis(a1,b1,a2,b2);
                    double s=acos((2*r*r-d*d)/(2*r*r));
                    cout<<"Case "<<i<<": "<<fixed<<setprecision(8)<<r*s<<"\n";
                    i++;
          }



          return 0;
}