UVA-438- The Circumference of the Circle

以下是本题的算法：

#include<stdio.h>#define PI 3.141592653589793int main(){    freopen("in.txt","r",stdin);    double x1,y1,x2,y2,x3,y3,c;    while(scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3)!=EOF)    {        double a2=pow(x1-x2,2)+pow(y1-y2,2);        double b2=pow(x2-x3,2)+pow(y2-y3,2);        double c2=pow(x3-x1,2)+pow(y3-y1,2);        double r2=4*a2*b2*c2/(pow(a2+b2+c2,2)-2*(a2*a2+b2*b2+c2*c2));        printf("%.2lf\n",PI*sqrt(r2));    }    return 0;}