CAT Important Formulae: Part 3
Candidates preparing for the CAT 2019 examination can expect 2- 3 questions from Permutation & Combination topic in the Quantitative Section. Here are some important formulae related to Permutation & Combination.
Let n be a positive integer. Then, factorial n denoted n! is defined as:
n! = n(n - 1)(n - 2) ... 3.2.1.
The different arrangements of a given number of things by taking some or all at a time, are called permutations.
Number of Permutations:
Number of all permutations of n things, taken r at a time, is given by:
nPr = n(n - 1)(n - 2) ... (n - r + 1) = n!
(n - r)!
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.
Number of Combinations:
The number of all combinations of n things, taken r at a time is:
nCr = n! = n(n - 1)(n - 2) ... to r factors
(r!)(n - r)! r!
nCn = 1 and nC0 = 1.
nCr = nC(n - r)