# Ruby – Find Cosine – cos() Method – Examples & Explanation

In Ruby, `cos()` is a method in `CMath` class which is used to find the cosine of any given complex number and this number must be in radians. To find out cosine of any given complex number `CMath.cos()` method is used.

## `cos()` Method in Ruby

`cos()` method exists in Standard CMath Library of Ruby. The purpose of this method is to provide the cosine of any given complex number and this number must be in radians.

## Syntax of the `cos()` Method in Ruby

Syntax of `cos()` method is:

```cmath.cos(x)
```

## Parameter of `cos()` Method in Ruby

 Parameter Description `x` Any valid complex number. This parameter is required.

Note: If the `x` parameter is not a complex number, `cos()` method will return an `error`.

## Return Value of `cos()`

`cos()` method will return the cosine of any given complex number and this number must be in radians.

## Ruby – Get the `cos()` of the Given Numbers

``` # import CMath library
require 'cmath'
x = CMath.cos(5 + 5i)
puts x
```

Output

` 21.050556181654844+71.1552598809822i`

## Ruby – Various Ways of Calculation in `cos()` Method

``` # import CMath library
require 'cmath'
x = CMath.cos(-10 + 5i)
puts x
```

Output

` -1.490839765215787+0.03976617365742172i`

## Ruby – Various Ways of Calculation in `cos()` Method

``` # import CMath library
require 'cmath'
x = CMath.cos(10 + 0i)
puts x
```

Output

` -0.8390715290764524+0.0i`

## Ruby – Non-Complex Nubmer in `cos()` Method Produces Error

``` # import CMath library
require 'cmath'
x = CMath.cos(a10 - b5i)
puts x
```

Output
` Error`

## Ruby – Get the `cos()` of Negative Number

``` # import CMath library
require 'cmath'
x = CMath.cos(-10)
puts x
```

Output

` -0.8390715290764524`