hdu 1374 The Circumference of the Circle

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求圆心公式：

             x0 = ((y3-y1)*(y2*y2-y1*y1+x2*x2-x1*x1)+(y2-y1)*(y1 *y1-y3*y3+x1*x1-x3*x3))/(2*(x2 -x1)*(y3-y1)-2*(x3-x1)*(y2-y1));y0= ((x3-x1)*(x2*x2-x1*x1 +y2*y2-y1*y1)+(x2-x1)*(x1*x1- x3*x3+y1*y1-y3*y3))/(2*(y2-y1)*(x3 -x1)-2*y -y1)*(x2-x1));

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#include<stdio.h>
#include <iostream>
#include<string.h>
#include<stdlib.h>
#include<math.h>
#include<algorithm>
#include<list>
#include<vector>
#pragma comment(linker,"/STACK:102400000,102400000")
using namespace std;
#define PI 3.141592653589793int main()
{double x1,x2,x3,y1,y2,y3,r;double x0,y0;while(scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3)!=EOF){x0 = ((y3 - y1)*(y2*y2-y1 * y1+x2*x2-x1*x1)+(y2-y1)*(y1 *y1-y3*y3+x1*x1-x3*x3))/(2*(x2 -x1)*(y3-y1)-2*(x3-x1)*(y2-y1));y0= ((x3-x1)*(x2*x2-x1*x1 +y2*y2-y1*y1)+(x2-x1)*(x1*x1- x3*x3+y1*y1-y3*y3))/(2*(y2-y1)*(x3 -x1)-2*y -y1)*(x2-x1));r=sqrt((x1-x0)*(x1-x0)+(y1-y0)*(y1-y0));printf("%.2lf\n",2*r*PI);}return 0;
}

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