一题水的不能再水的题目，秒了

#include <bits/stdc++.h>
using namespace std;
double a, b, c, p;int main(){
    cin >> a >> b >> c;
    p = (a + b + c) / 2;
    printf("%.3f", sqrt(p * (p - a) * (p - b) * (p - c)));
    //经典复刻——海伦公式
}

C :

#include<stdio.h>
#include<math.h>
#define S(x,y,z) (x+y+z)/2;
#define area(x,y,z,S) sqrt(S*(S-x)*(S-y)*(S-z))
main()
{
	double a,b,c,S;
	scanf("%lf%lf%lf",&a,&b,&c);
	S=S(a,b,c);
	printf("%.3f\n",area(a,b,c,S));
}

C++ :

#include<bits/stdc++.h>
using namespace std;
double a,b,c,x;
int main()
{
    cin>>a>>b>>c;
    x=(a+b+c)/2;
    printf("%.3f",sqrt(x*(x-a)*(x-b)*(x-c)));
    return 0;
}