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# atan

Arc tangent function of one variable
View other versions (4)

## Interface

#include <math.h>double atan (double x)long atanl (long double x)float atanf (float x)#include <complex.h>complex atan (complex x)double complex catan (double complex x)float complex catanf (float complex x)long double complex catanl (long double complex x)

## Description

The atan function computes the principal value of the arc tangent of x, returning a value in the range $\inline&space;[-\pi/2,&space;+\pi/2]$. The arc tangent is also often denoted as $\inline&space;\tan^{-1}(x)$.

The complex inverse tangent is defined by:
$\mathrm{atan}(x)&space;=&space;\frac{-1}{2}&space;i&space;\log&space;\left(&space;\frac{1+i&space;x}{1-i&space;x}&space;\right)$

Example:
##### Example - Arc tangent function of one variable
Workings
#include <stdio.h>
#include <math.h>
int main(void)
{
double result, x = 0.6325;
result = atan(x);
printf("The arc sin of %lf is %lf\n", x, result);
}
Solution

Output:

The arc sin of 0.632500 is 0.563974

## Special Values

atan ( ±0 ) returns ±0.

atan ( ±∞ ) returns ±$\inline&space;\pi/2$.

## Standards

The atan function conforms to ISO/IEC 9899:1999(E).

## Compatibility

DOSUNIXWindowsANSI CC++ only
atanlâ€¢â€¢
Real atanâ€¢â€¢â€¢â€¢
Complex atanâ€¢â€¢â€¢
Complex catanâ€¢â€¢