hdu1798几何公式

主要就是推公式了，我推了老半天，囧啊。。。还要注意一点就是内切的情况，开始没考虑，WA了一次。

/*
 * hdu1798/win.cpp
 * Created on: 2012-7-9
 * Author    : ben
 */
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <iostream>
#include <algorithm>
#include <queue>
#include <set>
#include <map>
#include <stack>
#include <string>
#include <vector>
#include <deque>
#include <list>
#include <functional>
#include <numeric>
#include <cctype>
using namespace std;

const double pi = acos(-1);

inline double mydistance(double x1, double y1, double x2, double y2) {
    return sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
}

int main() {
#ifndef ONLINE_JUDGE
    freopen("data.in", "r", stdin);
#endif
    double x1, y1, r1, x2, y2, r2;
    while(scanf("%lf%lf%lf%lf%lf%lf", &x1, &y1, &r1, &x2, &y2, &r2) == 6) {
        double d = mydistance(x1, y1, x2, y2);
        double ans;
        if(d >= r1 + r2) {//相离或外切
            ans = 0;
        }else  if(d <= fabs(r1 - r2)) {
                if(r1 <= r2) {
                    ans = pi * r1 * r1;
                }else {
                    ans = pi * r2 * r2;
                }
        }else {
            double x = (d * d + r2 * r2 - r1 * r1) / d / 2;
            double p1 = r1 * r1 * acos((d - x) / r1);
            double p2 = r2 * r2 * acos(x / r2);
            double p3 = d * sqrt(r2 * r2 - x * x);
            ans = p1 + p2 - p3;
        }
        printf("%.3f\n", ans);
    }
    return 0;
}