C++ - Hero's Method of Calculating Triangle Area

// tri_over.cpp
// Illustrate overloaded tri_area functions
#include 
#include 
#include 
#include 
using namespace std;

double tri_area (double base, double height);
// Base & Height formula
// Area = 1/2 x base x height

double tri_area (double a, double b, double c);
// Hero's Three Sides formula
// Area = Sqareroot [s(s-a)(s-b)(s-c)] where s=a+b+c (i.e. perimeter)

int main()
{
   double base, height,
         s1, s2, s3,
         area;
   cout << "Enter base and height of a triangle: ";
   cin >> base >> height;
   area = tri_area(base,height);
   cout << "Area = " << area << endl;
   cout << "Enter lengths of sides of a triangle: ";
   cin >> s1 >> s2 >> s3;
   cout << "Area = " << tri_area(s1, s2, s3) << endl;
   system("PAUSE");
   return 0;
}

double tri_area (double base, double height)

{double area_bh;
 area_bh=(base*height/2.0);
  return area_bh;
} // stub

double tri_area (double a, double b, double c)
{ double area_3s, per;
  per=((a+b+c)/2);
  area_3s=sqrt(per *(per-a)*(per-b)*(per-c));
  return area_3s;
}