C Program to find average area of circle using functions

/* 
   This program reports average area of circle using functions
*/

#include<stdio.h>
#include<math.h>

#define PI 3.1415926

double average(double, double) ;
double circleArea(double) ;

int main() {
   double outerDiameter, innerDiameter ;
   double avgArea ;

   printf("Outer Diameter = ") ;
   scanf("%lf", &outerDiameter) ;

   printf("Inner Diameter = ") ;
   scanf("%lf", &innerDiameter) ;

   avgArea 
     = average(circleArea(innerDiameter), 
	           circleArea(outerDiameter)
              ) ;

   printf("Average Area = %f\n", avgArea) ;
   
   return 0 ;
}


/* Calculate the average. */
double average(double value1, double value2) {
   double avgValue ;

   avgValue = (value1 + value2) / 2.0 ;

   return avgValue ;
}


/* Calculate the area of a circle. */
double circleArea (double diameter) {
   double area, radius ; 

   radius = diameter / 2.0 ;
   area = PI * radius * radius ;

   // printf("diameter = %f, radius = %f and area = %f\n", 
   //   diameter, radius, area) ;

   return area ;
}