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# How to use acos() in C++

Talha Ashar

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:

Figure 1 - Cos and acos functions

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>


## Parameters

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

## Return value

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:

Figure 2 - Return Types

## Example

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 radians
double x = 0.5;

// the result is also in radians
cout << "acos(0.5) = " << acos(x) << endl;

return 0;
}

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Talha Ashar