The **`cos()` function** returns the cosine of a number. To be more specific, it returns the cosine of a number in the radian float value. 

Figure 1 shows the mathematical representation of the `cos()` function.



>Note:
>- The `math.h` header file is required for this function.
>- This `cos()` function only works for right-angled triangles. 


# Syntax
```c
double cos(double num)
```
# Parameter
This function requires a `number` that represents an `angle` in `radians` as a parameter.

In order to convert `degrees` to `radians`, use the following formula:
```c
radians = degrees * ( PI / 180.0 )
```

# Return value
`cos()` returns the `cosine` of a number `(radians float value)` that is sent as a parameter.

# Example


#include<stdio.h>
//header file
#include<math.h>

int main() {
    //positive number in radians
    printf("The cosine of %lf is %lf \n", 2.3, cos(2.3));

    // negative number in radians
    printf("The cosine of %lf is %lf \n", -2.3, cos(-2.3));

    //converting the degrees angle into radians and then applying cos()
    // degrees = 90.0
    // PI = 3.14159265
    // result first converts degrees to radians then apply cos
    double result=cos(90.0 * (3.14159265 / 180.0));
    printf("The cosine of %lf is %lf \n", 90.0, result);


}

What is cos() in C?

The cos() function in C computes the cosine of a number in radians, requiring math.h.