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   acos
   Usage
   #include <math.h>
   double acos(double x);

   Description
   acos  returns the arc cosine of x in the range  of  0<=acos<=p.
   The argument x must be in the range of -1<=x<=1.

   Example
   #include <math.h>
   #include <stdio.h>
   main()
   {
   double x, y;
        x = .94;
        y = acos(x);
        printf("The arc cosine of %f is %f\n",x,y);
   }
   Return Value
   acos returns the arc cosine of the input value. If the value x is less
   than  -1 or greater than 1, acos sets errno to EDOM, prints  a  DOMAIN
   error message to stderr and returns 0. Error handling can be  modified
   through the function matherr.