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Q: factorials for addition? ( No Answer,   6 Comments )
Subject: factorials for addition?
Category: Science > Math
Asked by: lawboy-ga
List Price: $2.00
Posted: 17 Mar 2006 11:32 PST
Expires: 16 Apr 2006 12:32 PDT
Question ID: 708478
I know what a factorial is, but I can't remember whether there is a
word and/or formula taking a number, lets say 5, and adding 5+4+3+2+1,
which is 15.  Does this function (that's probably not the right word,
I'm not a mathematition, have a word, like "factorial"?  If so, what
is it?
There is no answer at this time.

Subject: answer
From: lophar-ga on 17 Mar 2006 13:00 PST
There is no special name for this function. But there is a simple formula:
1 + 2 + 3 + ... + n = n(n+1)/2 
The proof can be found here:

These numbers are sometimes called triangular:
Subject: Re: factorials for addition?
From: lawboy-ga on 17 Mar 2006 16:27 PST
Thanks lophar-ga.  Quick aside question.  It seems like many of the
easier questions on GA are answered for free, in the comments.  What's
your incentive?  ARe you trying to become a Researcher, or just a nice
guy who stumbled upon my question?  Or is there another explanation. 
Thanks again.
Subject: Re: factorials for addition?
From: ansel001-ga on 17 Mar 2006 19:00 PST
The motivation varies from person to person.  Some people hope to
become one of the official paid researchers, some just like to help,
and some like to show how smart they are.
Subject: Re: factorials for addition?
From: lophar-ga on 18 Mar 2006 16:21 PST
I answer questions just for fun when i have nothing else to do, and i
need no money for this.
Subject: Re: factorials for addition?
From: lawboy-ga on 23 Mar 2006 11:18 PST
OK.  Thanks to both of you.
Subject: Re: factorials for addition?
From: myoarin-ga on 10 Apr 2006 03:53 PDT
Your comment will probably be deleteed because you posted an email address.
That's no-no here.

Here is a definition of factorial:

"In mathematics, the factorial of a natural number n is the product of
the positive integers less than or equal to n. This is written as n!
and pronounced "n factorial". The notation n! was introduced by
Christian Kramp in 1808."

And here is another with an example:

"The product of a series of consecutive positive integers from 1 to a
given number (n). It is expressed with the symbol ( ! ). For example,
5! = 5x4x3x2x1 = 120. As a rule (n!+n) is evenly divisible by n."

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