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# Math: Combinatorics

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Re: Math: Combinatorics [#permalink]  04 Dec 2011, 07:17
...k objects taken from a set of n distinct objects....

so k is always less or equal N by definition.
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Re: Math: Combinatorics [#permalink]  14 Mar 2012, 16:34
Thank you for this post! Very helpful!
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Re: Math: Combinatorics [#permalink]  11 Apr 2012, 23:29
walker wrote:
The topic is not finished.

COMBINATORICS

created by: walker
edited by: bb, Bunuel

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This post is a part of [GMAT MATH BOOK]

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Circular arrangements

Let's say we have 6 distinct objects, how many relatively different arrangements do we have if those objects should be placed in a circle.

The difference between placement in a row and that in a circle is following: if we shift all object by one position, we will get different arrangement in a row but the same relative arrangement in a circle. So, for the number of circular arrangements of n objects we have:
R = \frac{n!}{n} = (n-1)!

Tips and Tricks

Any problem in Combinatorics is a counting problem. Therefore, a key to solution is a way how to count the number of arrangements. It sounds obvious but a lot of people begin approaching to a problem with thoughts like "Should I apply C- or P- formula here?". Don't fall in this trap: define how you are going to count arrangements first, realize that your way is right and you don't miss something important, and only then use C- or P- formula if you need them.

Resources

Walker's post with Combinatorics/probability problems: [Combinatorics/probability Problems]

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[Reveal] Spoiler: Images

 Attachment:Math_Combinatorics_6balls.png Attachment:Math_Combinatorics_6balls_b.png Attachment:Math_Combinatorics_6balls_l.png Attachment:Math_Comb_Round_t.png

having a hard time understand the formula under circular arrangement
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Re: Math: Combinatorics [#permalink]  04 Dec 2012, 20:26
just wanted to throw these videos from khan academy out there, super helpful in dumbing down the concepts and holding your hand through it

combinations:

permutations:

These should be required watching, MGMAT does a thorough job of confusing my ass in this area.
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Re: Math: Combinatorics [#permalink]  17 Mar 2013, 00:07
Great post., Many thanks!!
Re: Math: Combinatorics   [#permalink] 17 Mar 2013, 00:07
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