In trigonometry, the ** cosine** function of a right-angled triangle is the ratio between the adjacent and hypotenuse sides of the triangle.

The $cos^{-1}$ function is the inverse cosine of a value, as shown below:

The ** acos** function in C++ works exactly like the $cos^{-1}$ function in trigonometry and returns the inverse cosine of a value in

`radians`

.To use the `acos`

function, the `cmath`

library needs to be included in the program, as shown below:

```
#include <cmath>
```

The `acos`

function takes a single mandatory parameter, i.e., an integer or floating point number in the range [-1, 1].

Given that the argument is in the required range of [-1,1], the `acos`

function returns a value between $0$ and $π$.

If the argument is greater than $1$ or less than $-1$, then the `acos`

function returns $NaN$, i.e., not a number.

`acos`

can have the following three return types, depending on the the provided argument:

- float
- double
- long double

The figure below shows the return types in correspondence with different argument types that are passed to the `acos`

function:

The code below shows how the `acos`

function works in C++:

#include <iostream>#include <cmath>using namespace std;int main() {// the value of x is in radiansdouble x = 0.5;// the result is also in radianscout << "acos(0.5) = " << acos(x) << endl;return 0;}

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