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Why is 0!=1?

 

$${\mathtt{0}}{!} = {\mathtt{1}}$$

Guest Mar 2, 2015

Best Answer 

 #2
avatar+17655 
+10

0!  is defined to be  1  so that certain formula work.

For example: the formula for permutations is:  nPr  is  n! / (n - r)!  

     which wouldn't work for 5P5  =  5! / (5 - 5)!  =  5! / 0!  unless  0!  is defined to be  1.

A problem that requires  5P5  is:  How many different ways can you arrange 5 books on a bookshelf?

Since there are 5 ways to choose the first book, 4 ways to choose the second book, 3 ways to choose the third book, 2 ways to fourth books and 1 way to choose the last book, you get 5 x 4 x 3 x 2 x 1  =  120  =  5!.

geno3141  Mar 2, 2015
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5+0 Answers

 #1
avatar+78643 
+10

Not a proof....but notice....

3! = 4!/4

2!  = 3!/3

1! = 2!/2

So....this would imply that

0!  = 1! / 1  = 1/1 = 1

 

CPhill  Mar 2, 2015
 #2
avatar+17655 
+10
Best Answer

0!  is defined to be  1  so that certain formula work.

For example: the formula for permutations is:  nPr  is  n! / (n - r)!  

     which wouldn't work for 5P5  =  5! / (5 - 5)!  =  5! / 0!  unless  0!  is defined to be  1.

A problem that requires  5P5  is:  How many different ways can you arrange 5 books on a bookshelf?

Since there are 5 ways to choose the first book, 4 ways to choose the second book, 3 ways to choose the third book, 2 ways to fourth books and 1 way to choose the last book, you get 5 x 4 x 3 x 2 x 1  =  120  =  5!.

geno3141  Mar 2, 2015
 #3
avatar+91038 
+5

Thank you all.

This is a good question from an enquiring mind.

I liked both answers,  the 2 answers compliment each other really well I think.

Melody  Mar 3, 2015
 #4
avatar+78643 
+5

As an  addendum to geno's explanation.....suppose we don't want to choose any books from the shelf.....in other words...C(5,0)

The "formula" is

5! / [(5 - 0)! * 0!] = [5!/ (5! *  0!)]  = [1 / 0! ]

Well.....there 's only one way not to choose any books from the shelf.....don't take any of them!!!

Thus......the 0! in the "formula" MUST = 1  for this to "work"

 

CPhill  Mar 3, 2015
 #5
avatar+91038 
+5

I think I will add this thread to our reference material.  

Melody  Mar 3, 2015

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