Math Is Fun Forum

w0lfman

Member

Registered: 2005-05-16

Posts: 1

Factorials

Well, I know what factorials are, it is n*(n-1)*(n-2), etc. until you've gone all the way down to zero. What i'm looking for is a way to express n+(n-1)+(n-2), etc. Do you know of any way to write this?

Thanks, w0lfman

Zach

Member

Registered: 2005-03-23

Posts: 2,075

Re: Factorials

3n-3, I expect. I may be wrong.

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mathsyperson

Guest

Re: Factorials

(I think) this topic's been done before, but anyway...

You are asking how to express Triangular Numbers.

You can do this by using this formula:    n(n+1)
                                                         ---------
                                                              2

or also 0.5n² + 0.5n.

MathsIsFun

Administrator

Registered: 2005-01-21

Posts: 7,713

Re: Factorials

Yep, just recently covered here.

The question was adding up the numbers 1,2,3,4,5 (your problem is just like 5,4,3,2,1) but my answer included being able to add things like 5,6,7,8,9 (ie the general solution) ... and Milos rounded out the whole discussion.

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"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman