# atan C Library Function

The function double atan(double x); returns the arc tangent of x, expressed in radians. Arc tangent is the inverse operation of tangent.

## Function prototype of atan

double atan(double x);
• x : A floating point value whose arc tangent(inverse tangent) to be computed, in the interval [-INFINITY, INFINITY].

## Return value of atan

Function atan returns the principal arc tangent of x, in the interval [-Pi/2, +Pi/2] radians.

## C program using atan function

The following program shows the use of atan function to calculate inverse of tangent.

```#include <stdio.h>
#include <math.h>

#define PI 3.14159

int main(){
/* Range of tan function is -INFINITY to +INFINITY */
printf("Enter a number between -INFINITY to +INFINITY\n");
scanf("%lf", &input);

/*
*  One radian is equal to 180/PI degrees.
*/

printf("The arc tan of %0.4lf is %0.4lf in radian\n",
printf("The arc tan of %0.4lf is %0.4lf in degree\n",
input, degree);

return 0;
}
```

Output
```Enter a number between -INFINITY to +INFINITY
1.0
The arc tan of 1.0000 is 0.7854 in radian
The arc tan of 1.0000 is 45.0000 in degree
```
```Enter a number between -INFINITY to +INFINITY
9999.0
The arc tan of 9999.0000 is 1.5707 in radian
The arc tan of 9999.0000 is 89.9943 in degree
```