The **#key# `acos()` function:  also called the `arc cosine` function #key#** returns the inverse cosine of a number. To be more specific, it returns the inverse cosine of a number in  radians. 

Figure 1 shows the *mathematical representation* of the `acos()` function and Figure 2 shows the *visual representation* of the `acos()` function.

>Note: The `math.h` header file is required for this function.


To convert `radians` to `degrees`, use the following formula.
```c
degrees = radians * ( 180.0 / PI )
```

# Syntax
```c
double acos(double num)
```
# Parameter
This function requires a number as a parameter. The parameter must be a double value between -1 and 1.
- -1 <= parameter <= 1 

# Return value
`acos()` returns the `inverse cosine` of a number (radian double value) that is sent as a parameter. The return value lies in interval `[0, pi]` radians.

# Example


#include<stdio.h>
//header file
#include<math.h>

int main() {
    //positive number in radians
    printf("The arc cosine of %lf is %lf radians \n", 0.5, acos(0.5));

    // negative number in radians
    printf("The arc cosine of %lf is %lf radians \n", -0.5, acos(-0.5));

    //applying acos() and then converting the result in radians to degrees
    // radians = 0.5
    // PI = 3.14159265
    double result=acos(0.5) * (180.0 / 3.14159265);
    printf("The arc cosine of %lf is %lf degrees \n", 0.5, result);


}

What is acos() in C?

Returns the inverse cosine of a double between -1 and 1 using radians. Requires math.h.