【POJ2242】The Circumference of the Circle（初等几何）

已知三点坐标，算圆面积。

使用初等几何知识，根据海伦公式s = sqrt(p(p - a)(p - b)(p - c)) 和 外接圆直径 d = a * b * c / (2s) 来直接计算。

 #include <iostream>
 #include <cstring>
 #include <cstdlib>
 #include <cstdio>
 #include <numeric>
 #include <cctype>
 #include <cmath>
 #include <algorithm>

 #define PI acos(-1)
 using namespace std;

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

 int main () {
     ios :: sync_with_stdio(false);
     double x[], y[];
     while (cin >> x[] >> y[]) {
         for (int i = ; i < ; ++ i) {
             cin >> x[i] >> y[i];
         }
         double a, b, c;
         a = calc_dis (x[], y[], x[], y[]);
         b = calc_dis (x[], y[], x[], y[]);
         c = calc_dis (x[], y[], x[], y[]);
         //cout << a << " " << b << " " << c << endl;
         double p = (a + b + c) / ;
         double s = sqrt (p * (p - a) * (p - b) * (p - c));
         //cout << "s : " << s << endl;
         double d = a * b * c / ( * s);
         printf ("%.2f\n", PI * d);
     }
     return ;
 }