1386: [蓝桥杯2018决赛]三角形面积

#include<bits/stdc++.h>
using namespace std;
#define ll long long
double len(double x1,double y1,double x2,double y2)
{
    double l=sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
    return l;
}

int main()
{
    double a=len(2.3,2.5,6.4,3.1);
    double b=len(2.3,2.5,5.1,7.2);
    double c=len(6.4,3.1,5.1,7.2);

    double q=(a+b+c)/2;
    double sum=sqrt(q*(q-a)*(q-b)*(q-c));
    printf("%.03lf\n",sum);
}