An example of a C program where preprocessor directive #define and the mathematical function pow are employed.

preprocessor-example.c

/*C program to print area of a circle whose radius is given as input*/
#include <stdio.h>
#include <math.h>//for the function -> pow()

#define pi 3.141

int main(){
  double radius, area;
  printf("\nEnter the value of the radius: ");
  scanf("%lf",&radius);
  area = pi*pow(radius, 2);
  printf("\nThe area of the circle is : %lf", area);
return 0;
}