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：
1 #include <iostream>
2 #include <stdio.h>
3 #include <algorithm>
4 #include <math.h>
5 #define PI 2*asin(1) //**注意此处不能简单的用3.1415926，精度不够
6 #define length(x1, y1, x2, y2) sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1))
7 #define horn(a, b, c) ((a*a+b*b-c*c)/(2*a*b))
8 using namespace std;
9
10 int main(void){
11 double x1, y1, r1, x2, y2, r2;
12 while(scanf("%lf%lf%lf", &x1, &y1, &r1)!=EOF){
13 scanf("%lf%lf%lf", &x2, &y2, &r2);
14 double area;
15 double gg = length(x1, y1, x2, y2);
16 if(gg>=r1+r2){
17 area = 0;
18 }else if(fabs(r2-r1)>=gg){
19 area = PI*min(r2, r1)*min(r2, r1);
20 }else{
21 double horna = acos(horn(r1, gg, r2));
22 double hornb = acos(horn(r2, gg, r1));
23 double area1 = r1*r1*horna;
24 double area2 = r2*r2*hornb;
25 double area3 = 0.5*r1*r1*sin(horna*2);
26 double area4 = 0.5*r2*r2*sin(hornb*2);
27 area = (area1 - area3) + (area2 - area4);
28 }
29 printf("%.3lf
", area);
30 }
31 return 0;
32 }