The function **double atan2(double y, double x); ** returns the arc tangent of y/x, expressed in radians. Function atan2 takes into account the sign of both arguments in order to determine the quadrant.

## Function prototype of atan2

*double atan2(double y, double x);***y**: A floating point value representing an Y-coordinate.**x**: A floating point value representing an X-coordinate.

## Return value of atan2

Function atan2 returns the principal arc tangent of y/x, in the interval [-Pi, +Pi] radians.

## C program using atan2 function

The following program shows the use of atan2 function to calculate inverse tangent of y/x.

#include <stdio.h> #include <math.h> #define PI 3.14159 int main(){ double Y, X, radian, degree; printf("Enter value of Y and X\n"); scanf("%lf %lf", &Y, &X); radian = atan2(Y, X); /* * Radian to degree conversion * One radian is equal to 180/PI degrees. */ degree = radian * (180.0/PI); printf("The arc tan2 of %0.4lf and %0.4lf is %0.4lf radian\n", Y, X, radian); printf("The arc tan2 of %0.4lf and %0.4lf is %0.4lf degree\n", Y, X, degree); return 0; }

Output

Enter value of Y and X 5 5 The arc tan2 of 5.0000 and 5.0000 is 0.7854 in radian The arc tan2 of 5.0000 and 5.0000 is 45.0000 in degree