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RedHat 9 (Linux i386) - man page for atan2 (redhat section 3)

ATAN2(3)						     Linux Programmer's Manual							  ATAN2(3)

NAME
atan2 - arc tangent function of two variables
SYNOPSIS
#include <math.h> double atan2(double y, double x);
DESCRIPTION
The atan2() function calculates the arc tangent of the two variables x and y. It is similar to calculating the arc tangent of y / x, except that the signs of both arguments are used to determine the quadrant of the result.
RETURN VALUE
The atan2() function returns the result in radians, which is between -PI and PI (inclusive).
CONFORMING TO
SVID 3, POSIX, BSD 4.3, ISO 9899
SEE ALSO
acos(3), asin(3), atan(3), cos(3), sin(3), tan(3) 1993-06-08 ATAN2(3)