著名五星上将麦克阿瑟曾经说过：

做不出来这道题的都是伞兵

#include<bits/stdc++.h>
using namespace std;
double dist(double x1,double y1,double x2,double y2){
	return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
}
int main(){
	struct coor{
		int x;
		int y;
	}point[3];
	for(int i=0;i<3;i++){
		cin>>point[i].x>>point[i].y;
	}
	double a,b,c,p,mj;
	a=dist(point[1].x,point[1].y,point[2].x,point[2].y);
	b=dist(point[2].x,point[2].y,point[3].x,point[3].y);
	c=dist(point[1].x,point[1].y,point[3].x,point[3].y);
	p=(a+b+c)/2;
	mj=sqrt(p*(p-a)*(p-b)*(p-c));
	printf("%.2lf",mj);
}

我就是伞兵