atan
returns a value in radians, in the
range of 
to .
SYNOPSIS
#include <math.h> double atan(double x); float atanf(float x);
DESCRIPTION
atan
computes the inverse tangent (arc tangent)
of the input value.
atanf is identical to atan, save that it operates on floats.
RETURNS
atan
returns a value in radians, in the
range of
to .
COMPLIANCE
atan
is ANSI C. atanf
is an extension.