学步园

题目：给定不共线3点求过这三点的圆的周长。

分析：简单题、计算几何。解方程即可。同UVa190

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

double calc( double x0, double y0, double x1, double y1, double x2, double y2 )
{
	double A1 = x1-x0,B1 = y1-y0,C1 = x1*x1-x0*x0+y1*y1-y0*y0;
	double A2 = x2-x0,B2 = y2-y0,C2 = x2*x2-x0*x0+y2*y2-y0*y0;
	double c  = (B1*C2-B2*C1)/(A1*B2-A2*B1);
	double d  = (A2*C1-A1*C2)/(A1*B2-A2*B1);
	double e  = -x1*x1-y1*y1-x1*c-y1*d;
	double h  = c/-2.0;
	double k  = d/-2.0;
	double r  = sqrt((x1-h)*(x1-h)+(y1-k)*(y1-k));
	
	return 2.0*r*acos(-1.0);
}

int main()
{
	double x1,x2,x3,y1,y2,y3;
	while ( scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3) != EOF )
		printf("%.2lf\n",calc( x1, y1, x2, y2, x3, y3 ));
	return 0;
}