So you know how 5! is 5*4*3*2*1 right?

Then 4! is 4*3*2*1, or you could also say it's 5!/5

now let's go further down.

3! = 4!/4

2! = 3!/3

1! = 2!/2

0! = 1!/1

Now what is (-1)! ?

Then 4! is 4*3*2*1, or you could also say it's 5!/5

now let's go further down.

3! = 4!/4

2! = 3!/3

1! = 2!/2

0! = 1!/1

Now what is (-1)! ?