Factorial
Factorial of a number n is denoted as n! and is defined as the product of all positive integers less then or equal to n. i.e,
n! = n(n-1)(n-2).......3.2.1
Permutation
The different arrangements of a given number of things by taking some or all at a time are called as Permutation.
\[_{r}^{n}\textrm{P} = n(n-1)(n-2).....(n-r+1)= \frac{n!}{(n-r)!} \]
Combination
Each of the different groups or selections which can be formed by taking some or all of a number of objects , is called a Combination.
\[_{r}^{n}\textrm{C} = \frac{n!}{(r!)(n-r)!} \]
Note that : \[_{n}^{n}\textrm{C} =1 \] and \[_{n}^{0}\textrm{C} =1 \]
