/* P-2.2 */
/* Calculate the area and circumference of a circle
   of specified radius */


#include <stdio.h>
#define PI 3.14159

int main(void)
{
  double radius,        /* input - radius of a circle, cm         */
	 area,          /* output - area of a circle, cm^2        */
	 circumference; /* output - circumference of a circle, cm */
  /* Get the radius. */

  printf("Enter the radius in cm: ");
  scanf("%lf",&radius);

  /* Calculate the area and circumference. */

  area = PI*radius*radius;
  circumference = 2*PI*radius;

  /* Display the output. */

  printf("The area is %f cm^2.\n",area);
  printf("The circumference is %f cm.\n",circumference);
  return (0);
}

Sumber : David R. Brooks C Programming: The Essentials for Engineers and Scientists. Springer, NY, 1999