单纯的是为存模板 QAQ
#include<cstdio>
#include<cmath>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
struct Point{
double x,y;
Point(double x = 0,double y = 0):x(x),y(y){};
};
//两点的距离
double DistancePoint(Point a,Point b){
double x = a.x - b.x;
double y = a.y - b.y;
return sqrt(x * x + y * y);
}
typedef Point Vector;
//-----------------------------------------------------------------------
Vector operator +(Vector A,Vector B){return Vector(A.x + B.x,A.y + B.y);}
Vector operator -(Vector A,Vector B){return Vector(A.x - B.x,A.y - B.y);}
Vector operator *(Vector A,double p){return Vector(A.x * p,A.y * p);}
Vector operator /(Vector A,double p){return Vector(A.x / p,A.y / p);}
bool operator < (const Point &a,const Point &b){
return a.x < b.x || (a.x == b.x && a.y < b.y);
}
const double eps = 1e-10;
int dcmp(double x){
if(fabs(x) < eps) return 0; else return x < 0 ? -1 : 1;
}
bool operator == (const Point& a,const Point &b){
return dcmp(a.x - b.x) == 0 && dcmp(a.y - b.y) == 0;
}
//----------------------------------------------------------------------------
//点乘积 计算角度
double Dot(Vector A,Vector B){return A.x * B.x + A.y * B.y;}
double Length(Vector A){return sqrt(Dot(A,A));}
double Angle(Vector A,Vector B){
return acos(Dot(A,B) / Length(A) / Length(B));
}
//----------------------------------------------------------------------------
//叉积 面积的2倍
double Cross(Vector A,Vector B){return A.x * B.y - A.y * B.x;}
double Area2(Point A,Point B,Point C){
return Cross(B - A,C - A);
}
//向量旋转
Vector Rotate(Vector A,double rad){
return Vector(A.x * cos(rad) - A.y * sin(rad),A.x * sin(rad) + A.y * cos(rad));
}
//求法线
Vector Normal(Vector A){
double L = Length(A);
return Vector(- A.y / L, A.x / L);
}
//--------------------------------------------------------------------------------
//求交点
Point GetLineIntersection(Point P,Vector v,Point Q,Vector w){
Vector u = P - Q;
double t = Cross(w,u) / Cross(v,w);
return P + v * t;
}
//点到直线 P到A B
double DistanceToLine(Point P,Point A,Point B){
Vector v1 = B - A,v2 = P - A;
return fabs(Cross(v1,v2)) / Length(v1);
}
//点到线段
double DistanceToSegment(Point P,Point A,Point B){
if(A == B) return Length(P - A);
Vector v1 = B - A,v2 = P - A, v3 = P - B;
if(dcmp(Dot(v1,v2)) < 0) return Length(v2);
else if(dcmp(Dot(v1,v3)) > 0) return Length(v3);
else return fabs(Cross(v1,v2)) / Length(v1);
}
//点在直线上的投影
Point GetLineProjection(Point P,Point A,Point B){
Vector v = B - A;
return A + v * (Dot(v,P - A) / Dot(v,v));
}
//相交判定
bool SegmentProperIntersection(Point a1,Point a2,Point b1,Point b2){
double c1 = Cross(a2 - a1,b1 - a1), c2 = Cross(a2 - a1,b2 - a1);
double c3 = Cross(b2 - b1,a1 - b1), c4 = Cross(b2 - b1,a2 - b1);
return dcmp(c1) * dcmp(c2) < 0 && dcmp(c3) * dcmp(c4) < 0;
}
//允许在端点相交
bool OnSegment(Point p,Point a1,Point a2){
return dcmp(Cross(a1 - p,a2 - p)) == 0 && dcmp(Dot(a1 - p,a2 - p)) < 0;
}
//------------------------------------------------------------------------------
double x[3],y[3];
double pi = acos(-1.0);
//求垂直向量
Vector Solve(Vector v){
return Vector(-v.y,v.x);
}
int main(){
while(scanf("%lf%lf",&x[0],&y[0]) != EOF){
scanf("%lf%lf%lf%lf",&x[1],&y[1],&x[2],&y[2]);
double x1 = (x[0] + x[1]) / 2;
double y1 = (y[0] + y[1]) / 2;
double x2 = (x[0] + x[2]) / 2;
double y2 = (y[0] + y[2]) / 2;
Vector v1 = Vector(x[0] - x[1],y[0] - y[1]);
Vector v2 = Vector(x[0] - x[2],y[0] - y[2]);
Vector v3 = Solve(v1);
Vector v4 = Solve(v2);
//printf("%f %f %f %f\n",x1,y1,x2,y2);
//printf("%f %f %f %f\n",v3.x,v3.y,v4.x,v4.y);
Point z = GetLineIntersection(Point(x1,y1),v3,Point(x2,y2),v4);
//printf("%f %f\n",z.x,z.y);
double r = DistancePoint(z,Point(x[0],y[0]));
//printf("%f\n",r);
double d = pi * r * 2;
printf("%.2f\n",d);
}
return 0;
}
438 - The Circumference of the Circle【几何】
原文：http://blog.csdn.net/u013451221/article/details/44624135