I want to take a number and then do a sum similiar to a factorial.

Example:

In cell A1 I input 5

then in cell A2 the function adds 5+4+3+2+1

if I put in 3 it adds 3+2+1

is there a function that does this?

=a1*(a1+1)/2

It's a neat formula that some kid figured out a long time ago...

Say you want to add 1+2+3+4+5+6+...+100
Call that Sum S

We can write it two ways:
S = 1 + 2 + 3 + ... + 100
S = 100 + 99 + 98 + ... + 1

We add those two lines and we get:

2*S = 101 + 101 + 101 + ... + 101 (100 times)
so
2*S = 100 * 101

S = 100 * 101 / 2

Replace that 100 with an arbitrary N:

S = N*(N+1)/2



Tim wrote:

I want to take a number and then do a sum similiar to a factorial.

Example:

In cell A1 I input 5

then in cell A2 the function adds 5+4+3+2+1

if I put in 3 it adds 3+2+1

is there a function that does this?

--

Dave Peterson

The kid was Issac Newton. The story is that as punishment the teacher told
the whole class to add up the numbers from 1 to 100. Newton stared at it a
bit and just wrote down the answer. You can imagine the teacher's
response.

Bill
--------------------
On Wed, 22 Feb 2006 12:21:03 -0600, Dave Peterson wrote:

=a1*(a1+1)/2

It's a neat formula that some kid figured out a long time ago...

Say you want to add 1+2+3+4+5+6+...+100
Call that Sum S

We can write it two ways:
S = 1 + 2 + 3 + ... + 100
S = 100 + 99 + 98 + ... + 1

We add those two lines and we get:

2*S = 101 + 101 + 101 + ... + 101 (100 times)
so
2*S = 100 * 101

S = 100 * 101 / 2

Replace that 100 with an arbitrary N:

S = N*(N+1)/2

Tim wrote:

I want to take a number and then do a sum similiar to a factorial.

Example:

In cell A1 I input 5

then in cell A2 the function adds 5+4+3+2+1

if I put in 3 it adds 3+2+1

is there a function that does this?

I think it was Carl Friedrich Gauss, actually.

Bill Martin wrote:

The kid was Issac Newton. The story is that as punishment the teacher told
the whole class to add up the numbers from 1 to 100. Newton stared at it a
bit and just wrote down the answer. You can imagine the teacher's
response.

Bill
--------------------
On Wed, 22 Feb 2006 12:21:03 -0600, Dave Peterson wrote:

=a1*(a1+1)/2

It's a neat formula that some kid figured out a long time ago...

Say you want to add 1+2+3+4+5+6+...+100
Call that Sum S

We can write it two ways:
S = 1 + 2 + 3 + ... + 100
S = 100 + 99 + 98 + ... + 1

We add those two lines and we get:

2*S = 101 + 101 + 101 + ... + 101 (100 times)
so
2*S = 100 * 101

S = 100 * 101 / 2

Replace that 100 with an arbitrary N:

S = N*(N+1)/2

Tim wrote:

I want to take a number and then do a sum similiar to a factorial.

Example:

In cell A1 I input 5

then in cell A2 the function adds 5+4+3+2+1

if I put in 3 it adds 3+2+1

is there a function that does this?

--

Dave Peterson

http://www.newton.dep.anl.gov/askasc.../math99224.htm


--
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Even though the link has Newton in its address, it still refers to Gauss <vbg.

icestationzbra wrote:

http://www.newton.dep.anl.gov/askasc.../math99224.htm

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--

Dave Peterson

You're absolutely right! Somehow I've apparently had that wrong for 30
years. Makes me wonder what else I "know that isn't true?

Thanks.

Bill
-----------------------
On Wed, 22 Feb 2006 14:22:42 -0600, Dave Peterson wrote:

I think it was Carl Friedrich Gauss, actually.

Bill Martin wrote:

The kid was Issac Newton. The story is that as punishment the teacher told
the whole class to add up the numbers from 1 to 100. Newton stared at it a
bit and just wrote down the answer. You can imagine the teacher's
response.

Bill

It gets worse, Bill........believe me........<g

Vaya con Dios,
Chuck, CABGx3



"Bill Martin" wrote:

You're absolutely right! Somehow I've apparently had that wrong for 30
years. Makes me wonder what else I "know that isn't true?

Thanks.

Bill
-----------------------
On Wed, 22 Feb 2006 14:22:42 -0600, Dave Peterson wrote:

I think it was Carl Friedrich Gauss, actually.

Bill Martin wrote:

The kid was Issac Newton. The story is that as punishment the teacher told
the whole class to add up the numbers from 1 to 100. Newton stared at it a
bit and just wrote down the answer. You can imagine the teacher's
response.

Bill

"Tim" wrote:
I want to take a number and then do a sum similiar to a factorial.
Example:
In cell A1 I input 5
then in cell A2 the function adds 5+4+3+2+1
if I put in 3 it adds 3+2+1
is there a function that does this?

Function? I don't know. But there is a well-known formula
for this sum. In Excel, it would be:

=A1*(A1+1)/2

In B1, put formula =FACT(A1)

HTH,

--
AP

"Tim" a écrit dans le message de
...
I want to take a number and then do a sum similiar to a factorial.

Example:

In cell A1 I input 5

then in cell A2 the function adds 5+4+3+2+1

if I put in 3 it adds 3+2+1

is there a function that does this?

=SUMPRODUCT(--(ROW(INDIRECT("1:"&A1))))

--
HTH

Bob Phillips

(remove nothere from email address if mailing direct)

"Tim" wrote in message
...
I want to take a number and then do a sum similiar to a factorial.

Example:

In cell A1 I input 5

then in cell A2 the function adds 5+4+3+2+1

if I put in 3 it adds 3+2+1

is there a function that does this?