1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the difference between the Permutations and combinations

  1. Apr 22, 2010 #1
    Hi



    I want simple explanation of the Permutations and combinations and wich one has condition and I want simple example to undersand it



    I want your help
     
  2. jcsd
  3. Apr 22, 2010 #2

    statdad

    User Avatar
    Homework Helper

    At the simplest: permutations are used to count arrangements (order is important) and combinations are used when order does not matter.

    In neither case (again, in simplest examples) is it allowed to repeat items.

    Example: Consider the four letters A, B, C, D

    Question 1: How many different arrangements of three letters are there (repetitions not allowed)

    By listing we get:
    ABC, ACB, ABD, ADB, ACD,ADC
    BAC, BCA, BAD, BDA, BCD, BDC
    CAB, CBA, CBD, CDB, CAD, CDA
    DAB, DBA, DAC, DCA, DBC, DCB

    If you count there are 24 of these (they are considered different because of the different orderings). The long phrase is this:
    "There are 24 permutations of four letters selected three at a time"

    While there is no universal standard for the mathematical notation, the ones I've listed below are the most common.

    [tex]
    P(4,3) = 24, \quad P^4_3 = 24, {}^4P_3 = 24
    [/tex]

    The general formula for calculation can be written as

    [tex]
    P(m,n) = \frac{m!}{(m-n)!} = m \cdot (m-1) \cdot \dots \cdot (m - n + 1)
    [/tex]

    where [itex] m [/itex] is the number of items from which you choose, and [itex] n [/itex] is
    the number of items selected.

    For combinations, order is not important, only the set of objects selected. Again, if you look at the letters A, B, C, D, the number of ways to select three at a time (no repetititions) is
    four - the selections are
    ABC, ABD, ACD, BCD

    In notation

    [tex]
    C(4,3) = 4, \quad {}^4C_3 = 4, C^4_3 = 4
    [/tex]

    and the general formula is

    [tex]
    C(m,n) = \frac{m!}{n!(m-n)!} = \frac{P(m,n)}{n!}
    [/tex]

    Hope this helps.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: What is the difference between the Permutations and combinations
  1. Permutation combination (Replies: 11)

Loading...