1. Factorial Notation!
n! reads "n factorial"
5! =
8! =
n! = n x (n-1) x (n - 2) x (n - 3)...
is a natural number
How about 0! = 1
Likewise, n! = n(n - 1)!
or 8! = 8 · 7!

2. Simplify without a calculator:
Show 5! + 4! = 6 · 4!
Show (k + 1)! + k! = (k + 2)k!