#include <bits/stdc++.h>
 using namespace std;
  int main() {
    double x1, y1, x2, y2, x3, y3;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    double r1 = sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
    double r2 = sqrt((x3 - x1) * (x3 - x1) + (y3 - y1) * (y3 - y1));
     double r3 = sqrt((x2 - x3) * (x2 - x3) + (y2 - y3) * (y2 - y3));
    double p = (r1 + r2 + r3) / 2.0; double s = sqrt(p * (p - r1) * (p - r2) * (p - r3)); printf("%.2f", s);
return 0; 
}

#include<bits/stdc++.h> using namespace std; int main(){ double x,y,xx,yy,xxx,yyy,a; cin>>x>>y>>xx>>yy>>xxx>>yyy; a=(xx-x)*(yyy-y); a=a/2; cout<<fixed<<setprecision(2)<<a; return 0; }

#include <bits/stdc++.h> using namespace std; int main() { double x1, y1, x2, y2, x3, y3; cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3; double r1 = sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1)); double r2 = sqrt((x3 - x1) * (x3 - x1) + (y3 - y1) * (y3 - y1)); double r3 = sqrt((x2 - x3) * (x2 - x3) + (y2 - y3) * (y2 - y3)); double p = (r1 + r2 + r3) / 2.0; double s = sqrt(p * (p - r1) * (p - r2) * (p - r3)); printf("%.2f", s); return 0; }

#include <stdio.h> #include <math.h> // 用于浮点数绝对值函数fabs

int main() { double x1, y1, x2, y2, x3, y3; // 读取6个双精度浮点数 scanf("%lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3); // 计算叉积的绝对值，再乘以0.5得到面积 double area = 0.5 * fabs( (x2 - x1) * (y3 - y1) - (y2 - y1) * (x3 - x1) ); // 输出保留两位小数 printf("%.2lf\n", area); return 0; }//关键点：向量叉乘