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acos()                   Calculate Arc Cosine

 #include   <math.h>

 double     acos(x);                     arc cosine (in radians)
 double     x;

    acos() returns the arc cosine of 'x' in radians.  The return value
    will be in the range 0 to pi.  'x' must be in the range -1.0 to 1.0.

    Returns:    The arc cosine of 'x'.  If 'x' < -1.0 or 'x' > 1.0, then
                matherr() is called with a DOMAIN error; 'errno' (defined
                in <stdlib.h>) is set to EDOM (defined in <math.h>); a
                domain error message is printed to 'stderr'; and 0.0 is
                returned.

      Notes:    Error handling can be modified by using the matherr()
                routine.

  -------------------------------- Example ---------------------------------

    The following statements continue printing the arc cosines of the
    values in an array until a DOMAIN error occurs.

           #include <math.h>       /* for acos() and EDOM */
           #include <stdio.h>      /* for printf() */
           #include <stdlib.h>     /* for errno */

                                /* the value 3.141 will cause an error */
           double cosines[6] = {0.707, 0.0, 1.0, -1.0, 3.141, -0.2};

           main()
           {
               double arc_cosine;
               int i;

               errno = 0;
               for (i = 0;  !errno && i < 6;  i++) {
                   arc_cosine = acos(cosines[i]);
                   if (errno == EDOM) /* an error is also printed to stderr
           */
                       printf("DOMAIN error in acos()\n");
                   printf("arc cosine of %f = %f radians\n",
                          cosines[i], arc_cosine);
               }
           }