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Here,

Angle by n segments is: 2∏  radian = 360 degree

Angle by one segment is, θ= 2∏/n  radian = 360/n degree
from the above figure we get,

h/r=sinθ

=>h=r*sinθ

Area of the triangle =Area by one segment= (1/2)*r*h=0.5*r*r*sinθ

Area by n segments =n*0.5*r*r*sinθ

#include<stdio.h>
#include<math.h>
 
int main()
{
    double n,r,areaOne,areaAll,theta;
    while(scanf("%lf%lf",&r,&n)==2)
    {
        theta=2*acos(-1);
        theta=theta/n;
        areaOne=0.5*r*r*sin(theta);
        areaAll=areaOne*n;
        printf("%.3lf\n",areaAll);
    }
    return 0;
}

 

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