Formulas To Solve Permutation Combination

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Permutation and Combination Formulas & Definitions

Permutation:

The different arrangements of a given number of things by taking some or all at a time, are called permutations. This is denoted by nPr.

  • Permutations are studied in almost every branch of mathematics, and in many other fields of science. In computer science, they are used for analyzing sorting algorithms.

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. This is denoted by ncr.

permutation and combination formulas
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Formula for Permutation and Combination

  • Number of all permutations of n things, taken r at a time, is given by
    nPr = n!/(n-r)!
  • Number of all combinations of n things, taken r at a time, is given by
    nCr = n!/(r!) (n-r)!

Points to remember

  • Factorial of any negative quantity is not valid.
  • If a particular thing can be done in m ways and another thing can be done in n ways, then either one of the two can be done in m + n ways and both of them can be done in m × n ways
  • 0! = 1
  • 1! = 1
  • If from the total set of n numbers p is of one kind and ‘q’ and ‘r’ are others respectively then nPr = n! / p! × q! × r!.
  • nPn = n!
  • ncn = 1
  • nc0 = 1
  • ncr = nc(n-r)
  • nc0+nc1+nc2+nc3+…ncn = 2n
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Permutation and Combinations Formulas- Factorial

n ! = n(n-1)(n-2) …… 1

Eg. – 5! = 5(5-1)(5-2)(5-3)(5-4) = 5(4)(3)(2)(1)

Standard Truths

  • 0! = 1
  • n! only exists of n >= 0 and doesn’t exist for n < 0
nn!
01
11
22
36
424
5120
6720
75 040
840,320
9362 880
103 628 800
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Permutations Formulas

Number of ways in which Permutations out of n things r things can be SELECTED & ARRANGED (denoted by nPr ).

nPr = number of permutations (arrangements) of n things taken r at a time.

nPr = n!/ (n – r)!; n ≥ r
Eg.

  • Arrangement of Letters/Alphabets to form words with meaning or without meaning.
  • Arrangements of balls on a table.
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Formulas for COMBINATIONS

The number of ways in which r things at a time can be SELECTED from from n things is Combinations (represented by Cr)..

nCr = Number of combinations (selections) of n things taken r at a time.

  • nCr = n! /[r! (n–r)!] ; where n ≥ r (n is greater than or equal to r).

Eg.

  • Selections for people from total numbers who want to go out on a picnic.
  • Filling posts with people
  • Selection for a sports team out of available players
  • Selection of balls from a bag

Imp Properties:

  • nPr = r! ×nCr
  • Number of permutations (or arrangements) of n different things taken all at a time = n!
  • For Objects in which P1 are alike and are of one type, P2 are alike od different type and P3 are alike or another different type and the rest must be all different, Number of permutations = n! / P1 ! P2 ! P3!
  • When repetition is allowed number of permutations of n different things taken r at a time = n × n× n ×… (r times) = nr
  • Number of ways of distributing n identical things among r persons when each person may get any number of things = n + r – 1Cr–1
  • nCr + nCr-1 =n+1Cr
  • nCX =nCy when x = y or x + y = n
  • nCr =nPn-r
  • r . nCr = n .n-1Cr-1
  • nCr/ (r + 1) =n+1Cr+1/( n + 1)
  • For nCr to be greatest,
    • (a) if n is even, r = n/2
    • (b) if n is odd, r = (n + 1)/2 or (n – 1)/2