The factorial of a positive integer [tex]n[/tex], is the product of all positive integers less than or equal to [tex]n[/tex]. The factorial of [tex]n[/tex] is written as [tex]n![/tex].
By definition
[tex]2!=2\times1[/tex]
[tex]3!=3\times2\times1[/tex]
[tex]4!=4\times3\times2\times1[/tex]
Therefore
[tex]n!=n\times(n-1)\times(n-2)\times(n-4)...\times2\times1[/tex]
This can rewritten as
[tex]n!=n\times(n-1)![/tex]
