# C acos() library function

The function double acos(double x); returns the arc cosine of x, expressed in radians. Arc cosine is the inverse operation of cosine, It is also known as cosine inverse.

## Function prototype of acos

double acos(double x);
• x : A floating point value whose arc cosine(inverse cosine) to be computed, in the interval [-1, 1] (including -1 and 1).

## Return value of acos

Function acos returns the principal arc cosine of x, in the interval [0, PI] radians.

## C program using acos function

The following program shows the use of acos function to calculate inverse of cosine.

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

#define PI 3.14159

int main(){
double input, radian, degree;
/* Range of cosine function is from -1 to 1*/
printf("Enter a floating number between -1 and 1\n");
scanf("%lf", &input);

/*
*  Radian to degree conversion
*  One radian is equal to 180/PI degrees.
*/
degree = radian * (180.0/PI);

printf("The arc cosine of %0.4lf is %0.4lf in radian\n",
```Enter a floating number between -1 and 1