求三角形外接圆周长

#define PAI 3.1415926

#include <stdio.h>
#include <math.h>
//求三角形外接圆周长

//三角形三边a,b,c,半周长p（p=(a+b+c)/2） 
//面积：S= √[p(p - a)(p - b)(p - c)]
//外接圆半径 R=abc/4S 
//外接圆半径 R=abc/4√[p(p - a)(p - b)(p - c)] 
//外接圆周长C=2*PAI*R

float S(float x1,float y1,float x2,float y2,float x3,float y3){
    return ((x1-x3)*(y2-y3)   -   (y1-y3)*(x2-x3) );
}

float cal_center_x(float x1,float y1,float x2,float y2,float x3,float y3){
    return (S(x1*x1+y1*y1,y1, x2*x2+y2*y2, y2,x3*x3+y3*y3,y3)/(2*S(x1,y1,x2,y2,x3,y3)) );
}

float cal_center_y(float x1,float y1,float x2,float y2,float x3,float y3){
    return (S(x1, x1*x1+y1*y1, x2, x2*x2+y2*y2, x3, x3*x3+y3*y3) / (2*S(x1,y1,x2,y2,x3,y3)));
}

int main()
{
    float x1, y1, x2, y2, x3, y3, x, y;
    while(scanf("%f %f %f %f %f %f", &x1, &y1, &x2, &y2, &x3, &y3) != EOF)
    {
        x = cal_center_x(x1, y1, x2, y2, x3, y3);
        y = cal_center_y(x1, y1, x2, y2, x3, y3);
        float C = 2*PAI*sqrt((x-x1)*(x-x1)+(y-y1)*(y-y1));
        printf("%.2f\n", C);
    }    
    return 0;
}