acos() Math Function In C++ is a mathematical function that helps to find out the inverse cosine of a number.

The inverse cosine of a number in radian is computed with this function.

`acos(x) = cos -1x`

## Syntax:

Let’s say the number is x. The syntax would be as follows:

```
float acos(float x);
double acos(double x);
long double acos(long double x);
double acos(integral x);
```

## Parameter:

x: The value for which the arc cosine must be computed. It should fall between [-1,1].

## Return value:

Parameter | Return value |
---|---|

-1≤x≤1 | (0,∏) |

x<-1 or x>1 | Not a number |

### Example-1:

Let’s look at a basic case where x is higher than 1.

#include <iostream> #include<math.h> using namespace std; int main() { float degree=90; float x=degree*3.14/180; std::cout << "Value of cosine is :" <<cos(x)<< std::endl; cout<<"Inverse of cosine is :"<<acos(x); return 0; }

**Output:**

```
Value of cosine is :0.000796274
Inverse of cosine is :nan
```

In this case, the acos() function computes the inverse cosine of an integer when x is larger than 1.

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## Example-2:

Let’s look at a basic case where x is equal to zero.

#include <iostream> #include<math.h> using namespace std; int main() { float degree=0; float x=degree*3.14/180; std::cout << "Value of cosine is :" <<cos(x)<< std::endl; cout<<"Inverse of cosine is :"<<acos(x); return 0; }

**Output:**

```
Value of cosine is :1
Inverse of cosine is :1.5708
```

When the value of x is equal to zero, the acos() function computes the inverse cosine of a number.

## Example-3

Let’s have a look at a basic case where x is smaller than -1.

#include <iostream> #include<math.h> using namespace std; int main() { float degree= -60; float x=degree*3.14/180; std::cout << "Value of cosine is :" <<cos(x)<< std::endl; cout<<"Inverse of cosine is :"<<acos(x); return 0; }

**Output:**

```
Value of cosine is :0.50046
Inverse of cosine is :nan
```

In this case, the acos() function computes the inverse cosine of an integer when x is less than -1.