Here is a program that computes the arithmetic-geometric mean of two
numbers:
   
   #include <stdio.h>
   #include <math.h>

   int main()
   {
     double a;
     double b;
     double c;
     double d;
     int i;

     scanf("%lf%lf",&a,&b);
     for (i = 0;i < 1000;++i) {
       c = (a + b)/2;
       d = sqrt(a * b);
       a = (c + d)/2;
       b = sqrt(c * d);
     }
     printf("%lf\n",a);

     return 0;
   }

Here is an incomplete second version of the program:

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

   int main()
   {
     double a;
     double b;
     double c;
     double d;
     int i;

     scanf("%lf%lf",&a,&b);
     for (i = 0;i < 1000;++i) {
       averages(a,b,&c,&d);
       averages(c,d,&a,&b);
     }
     printf("%lf\n",a);

     return 0;
   }

Write an averages() function that makes the second version work just
like the first.