sin PI and other things

#include<stdio.h>
#include<cmath>
int main()
{
	double PI=2*acos(0.0);
	int cas;
	scanf("%d",&cas);
	for(int T=1;T<=cas;T++)
	{
		double R,r;
		double n;
		scanf("%lf%lf",&R,&n);
		double ang=PI/n;
		r=R*sin(ang)/(1+sin(ang));
		printf("Case %d: %.10lf\n",T,r);
	}
}