# C math library function atan()

In this article, you will learn about C math library function `atan()` that computes arc tangent with explanation and example.

`atan()` is the standard C math library function that is defined in math library `math.h`.

``#include<math.h>``

## Function prototype of C math library function atan()

``double atan( double x );``

where,

x = floating point value of any range

### Return Value of atan( )

This function returns the arc tangent (inverse tan) in radians of `x` that ranges in the interval [ -pi / 2, +pi / 2 ].

The return type is double.

## Example: Program to demonstrate the use of C math library function atan()

``````/*Use of math library function atan*/

#include<stdio.h>
#include<math.h>
#define PI 3.1415926

int main()
{
double x, y;

x = 0.5;
y = -1.5;

printf("atan(%.2lf) = %.2lf (radians)\n\n", x, atan( x ));

//calculation arc tan in degrees
printf("atan(%.2lf) = %.2lf (degree)\n\n", x, atan( x ) * 180 / PI );

printf("atan(%.2lf) = %.2lf (radians)\n\n", y, atan( y ));

//calculation arc tan in degrees
printf("atan(%.2lf) = %.2lf (degree)\n", y, atan( y ) * 180 / PI );

return 0;
}
``````

Output

Explanation

In the above program, we have calculated the arc tan of `x` and `y` in radian and degree.

We have defined macro `PI` for representing the value of `pi`.