[Blue Bridge Cup] The area of the triangle in the 2018 final (Helen's formula for the area of the triangle)

Title description

It is known that the coordinates of the three vertices of the triangle in the Cartesian coordinate system are:

(2.3, 2.5)
(6.4, 3.1)
(5.1, 7.2)

Find the area of ​​the triangle.
The output is
required to be accurate to 3 decimal places. If it is less than 3 digits, zero should be added.

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Helen's formula:

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Code:

#include <iostream>
using namespace std;

double len(double x1,double y1,double x2,double y2){
    
     //两点距离
	return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
}

int main() 
{
    
    
	double a,b,c,p,s;
	a = len(2.3,2.5,6.4,3.1); //三条边长 a b c
	b = len(2.3,2.5,5.1,7.2); 
	c = len(6.4,3.1,5.1,7.2);
	p = (a+b+c)/2; //半周长
	s = sqrt(p*(p-a)*(p-b)*(p-c)); //面积公式
	printf("%.3f",s); //保留三位
	return 0;
}