I know that . If you don't, here, I show you.
We use (or factorial) like this:
Here's an example:
Another way to do this is this. For example, .
Okay now, why ? Let's start from to .
(already did the )
Okay, now... ...
Surprising right? Okay, now you know.
What I want to talk about is that why are we allowed to do ? Because if we are, that mean and that's wrong.
Can anyone tell me about this? I'm always thinking about this.