Expert: Scott A Wilson - 10/8/2011

Question
Sir I am an eleventh standard student and having difficulty in solving questions on permutations and combinations.Also I am confused where to use permutation and where to use combination.plzzzz give a clear explanation about these

Answer
A permuation involves factorials.  In other words, P(a,b) = a!/(b!(a-b)!).
An example would be P(5,2) = 5!/(2!(5-2)!).  That reduces to 5!/(2!3!).
Since the ! symbol means multiplying by everything between the number and 1, this is
5*4*3*2*1/(2*1 * 3*2*1).  Note that the 3*2*1 occurs in the numerator and the denominator,
so this is 5*4/(2*1).  Muliplying this out gives 20/2, which is 10.
This is used when we have care of the order in which the objects are chosen.

A combination is just C(a,b) = a!/b!.  For example, if we had 5 items and we were to choose 2 of them, the number of ways to choose them when worrying about order would be
C(5,2) = 5!/2! = 5*4*3*2*1/(2*1) = 5*4*3 = 60.