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  Include:   <math.h>, <errno.h>
 
  Syntax:    double acos( double x );
             double asin( double x );
             double atan( double x );
             double atan2( double y, double x );
 
  Returns:   (acos) the arccosine result, or 0 if x is greater than
                    1 or less than -1.
             (asin) the arcsine result, or 0 if x is greater than 1 or
                    less than -1.
             (atan) the arctangent result.
             (atan2) the arctangent result, or 0 if both arguments
                     are 0.
             errno: EDOM
 
  See also:  cos, matherr, sin, tan
                                    -♦-