I have a pandas dataframe and a list as follows.

```
cols = ['n1', 'n2']

item n1 n2 n3
item1 1 2  5
item2 2 3  3
item3 1  1  1

```

I want to multiply the selected columns in `cols` and add it as a seperate column in the dataframe. i.e.,

```
item n1 n2 n3 multiplication
item1 1 2  5        2
item2 2 3  3        6
item3 1  1  1       1

```

I tried to do this using `multiply` function as follows.
`df[n1].multiply(df[n2])`

However, this is easy when there is only 2 `columns`. Suppose, I have many columns in `cols` list (e.g., `cols = ['n1', 'n2', 'n3', 'n4', 'n5']`). How to do such multiplications efficiently in pandas? maybe using one line of code.

I am happy to provide more details if needed.