#include <iostream.h>
// Use the mathematics library, which contains a declaration for M_PI:
#include <math.h>
// Then, define tank_car_volume, a function that uses M_PI:
double tank_car_volume (double r, double l) {
  return M_PI * r * r * l;
}
// Then, define main:
main ( ) {
  cout << "The volume of a standard tank car is "
       << tank_car_volume (3.5, 40.0) << endl;
}