Factorial

n!= 1 x 2 x 3 x ....   x n

0!=1
1!=1


2!= 1 x 2
3!=  1 x 2 x 3
4!=  1 x 2 x 3 x 4
5!=  1 x 2 x 3 x 4 x 5
6!=  1 x 2 x 3 x 4 x 5 x 6
7! =  1 x 2 x 3 x 4 x 5 x 6 x 7
8! =  1 x 2 x 3 x 4 x 5 x 6 x 7 x 8
9! =  1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9
10! = 1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9 x 10


10! = 9! x 10
      = 10 x 9 x 8!
     = 10 x 9 x 8 x 7!

n!  = n x (n-1)!
     = n x (n-1) x (n-2)!