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Difference between revisions of "Combination"

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* [http://www.artofproblemsolving.com/Forum/resources.php?c=182&cid=45&year=2005&p=368223 AIME 2005II/1]
 
* [http://www.artofproblemsolving.com/Forum/resources.php?c=182&cid=45&year=2005&p=368223 AIME 2005II/1]
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* [http://www.artofproblemsolving.com/Forum/resources.php?c=182&cid=45&year=2000&p=385889 AIME 2000II/5]
  
 
=== See also ===
 
=== See also ===

Revision as of 11:42, 19 June 2006

Definition

The number of combinations of ${r}$ objects from a set of ${n}$ objects is the number of ways the ${r}$ objects can be arranged with regard to order.

Notation

The common forms of denoting the number of combinations of ${r}$ objects from a set of ${n}$ objects is:

  • ${n}\choose {r}$
  • ${C}(n,r)$
  • $\,_{n} C_{r}$

Formula

${{n}\choose {r}} = \frac {n!} {r!(n-r)!}$

Derivation

Consider the set of letters A, B, and C. There are $3!$ different permutations of those letters. Since order doesn't matter with combinations, there is only one combination of those three. In general, since for every permutation of ${r}$ objects from ${n}$ elements $P(n,r)$, there are ${r}!$ more ways to permute them than to choose them. We have ${r}!{C}({n},{r})=P(n,r)$, or ${{n}\choose {r}} = \frac {n!} {r!(n-r)!}$.


Examples

See also

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