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 showing the use of 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 = acos(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", input, radian); printf("The arc cosine of %0.4lf is %0.4lf in degree\n", input, degree); return 0; }

**Program Output**

Enter a floating number between -1 and 1 0.5 The arc cosine of 0.5000 is 1.0472 in radian The arc cosine of 0.5000 is 60.0001 in degree