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 to show the use of 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(){ double input, radian, degree; /* Range of tan function is -INFINITY to +INFINITY */ printf("Enter a number between -INFINITY to +INFINITY\n"); scanf("%lf", &input); radian = atan(input); /* * Radian to degree conversion * One radian is equal to 180/PI degrees. */ degree = radian * (180.0/PI); printf("The arc tan of %0.4lf is %0.4lf in radian\n", input, radian); printf("The arc tan of %0.4lf is %0.4lf in degree\n", input, degree); return 0; }

**Program 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