ex19.cpp

// Solution to Exercise 19
// (NDE, 2013-01-19)
#include <iostream>
#include <cmath>
using namespace std;
double triangle_area(double a, double b, double c)
{
  double s = (a + b + c) / 2;
  double area = sqrt(s*(s-a)*(s-b)*(s-c));
  return area;
}
int main()
{
  double side1, side2, side3;
  cout << "Enter length of side 1: ";
  cin >> side1;
  cout << "Enter length of side 2: ";
  cin >> side2;
  cout << "Enter length of side 3: ";
  cin >> side3;
  double area = triangle_area(side1, side2, side3);
  cout << "Area = " << area << endl;
  return 0;
}

Note how variables side1, side2, side3 are used as arguments in the function call, whereas the function defines parameters named a, b, c. The names of variables used as arguments do not need to be the same as the names of corresponding parameters.