hdu1798(圆的位置关系)

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1798

题意：给出两个圆的圆心坐标与半径，求他们相交部分的大小

思路：有三种情况；

1. 两圆相离，area = 0；

2. 两圆内涵，area = PI×min(r1, r2)×min(r1, r2)；

3. 一般情况相交，用扇形面积减去三角形面积；

代码：

 #include <iostream>
 #include <stdio.h>
 #include <algorithm>
 #include <math.h>
 #define PI 2*asin(1)   //**注意此处不能简单的用3.1415926，精度不够
 #define length(x1, y1, x2, y2) sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1))
 #define horn(a, b, c) ((a*a+b*b-c*c)/(2*a*b))
 using namespace std;

 int main(void){
     double x1, y1, r1, x2, y2, r2;
     while(scanf("%lf%lf%lf", &x1, &y1, &r1)!=EOF){
         scanf("%lf%lf%lf", &x2, &y2, &r2);
         double area;
         double gg = length(x1, y1, x2, y2);
         if(gg>=r1+r2){
             area = ;
         }else if(fabs(r2-r1)>=gg){
             area = PI*min(r2, r1)*min(r2, r1);
         }else{
             double horna = acos(horn(r1, gg, r2));
             double hornb = acos(horn(r2, gg, r1));
             double area1 = r1*r1*horna;
             double area2 = r2*r2*hornb;
             double area3 = 0.5*r1*r1*sin(horna*);
             double area4 = 0.5*r2*r2*sin(hornb*);
             area = (area1 - area3) + (area2 - area4);
         }
         printf("%.3lf\n", area);
     }
     return ;
 }