#include<iostream>
#include<math.h>
using namespace std;
int main()
{
     float a, b, c;
     cout<<"a=";cin>>a;
     cout<<"b=";cin>>b;
     cout<<"c=";cin>>c;
     if(a<=0||b<=0||c<=0) cout<<" Xin moi ban nhap lai"<<endl;
     if(a>0&&b>0&&c>0)
     {
           float p=(a+b+c)/2;
           float s=sqrt(p*(p-a)*(p-b)*(p-c));
           cout<<"dien tich cua tam giac la: S="<<s<<endl;
     }
     system("pause");
}
  

#include<iostream>
using namespace std;
struct mang
{
        int gt;
        int d;
};
int main()
{
     int n;
     cout<<"nhap vao so phan tu cua mang: n=";cin>>n;
     mang a[n];
     for(int i=0;i<n;i++)
     {
             cout<<"a["<<i<<"]=";cin>>a[i].gt;
             a[i].d=0;
     }
     for(int i=0;i<n;i++)
     {
             for(int j=0;j<n;j++)
             {
                             if(a[i].gt==a[j].gt)  
                             {
                    a[i].d++;
                    a[j].d++;
                             }
             }
     }
     int s=0;
     for(int i=0;i<n;i++)
     {
             if(a[i].d==2) s=s+a[i].gt;
     }
     cout<<"tong cac so xuat hien mot lan duy nhat la: s="<<s<<endl;
     system("pause");
}