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## #1 2010-02-15 07:19:46

iamfriendly
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### Seeking anwser for a challenging question !!

Here is the question:

what is the difference for the equation of  "square of 1 - square of 2 + square of 3 - square of 4........+ square of 99 - square of 100"?

I got -5500, is that correct?

Thank you for helping!!!

EYE AM FRIENDLY, THAT'S O U NEED 2 NO,
psst,
Don't trust strangers, EYE AM FRIENDLY, THAT'S O U NEED 2 NO ..........psst, Don't trust strangers......

## #2 2010-02-15 07:24:53

bobbym

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### Re: Seeking anwser for a challenging question !!

Hi iamfriendly;

I don't think that is correct, I am getting an answer of - 5050

In mathematics, you don't understand things. You just get used to them.
90% of mathematicians do not understand 90% of currently published mathematics.
I am willing to wager that over 75% of the new words that appeared were nothing more than spelling errors that caught on.

## #3 2010-02-15 07:32:47

iamfriendly
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### Re: Seeking anwser for a challenging question !!

thank u!!

i will double check but i think i m on the right track

EYE AM FRIENDLY, THAT'S O U NEED 2 NO,
psst,
Don't trust strangers, EYE AM FRIENDLY, THAT'S O U NEED 2 NO ..........psst, Don't trust strangers......

## #4 2010-02-15 07:34:02

iamfriendly
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### Re: Seeking anwser for a challenging question !!

Yep, you are correct, Thank you again!!

EYE AM FRIENDLY, THAT'S O U NEED 2 NO,
psst,
Don't trust strangers, EYE AM FRIENDLY, THAT'S O U NEED 2 NO ..........psst, Don't trust strangers......

## #5 2010-02-15 07:36:50

iamfriendly
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### Re: Seeking anwser for a challenging question !!

May I ask how did you got the answer, we probably used different methods?

EYE AM FRIENDLY, THAT'S O U NEED 2 NO,
psst,
Don't trust strangers, EYE AM FRIENDLY, THAT'S O U NEED 2 NO ..........psst, Don't trust strangers......

## #6 2010-02-15 07:48:54

bobbym

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### Re: Seeking anwser for a challenging question !!

Hi;

Of course you can ask. I do it the easy way.

There are 2 ways I work that type of problem when I need a quick rough and tumble answer.

!) I take it to wolfram alpha, a site that is available to everyone. It immediately provides the -5050 result.

2) I look it up in a table of sums that was provided to me when I needed them on a daily basis for problems.

Then just substitute in for n = 100 to get -5050.

Actually to do it analytically is not difficult at all, you can try differences and a generating function. Proving that your answer is correct would probably involve induction.

In mathematics, you don't understand things. You just get used to them.
90% of mathematicians do not understand 90% of currently published mathematics.
I am willing to wager that over 75% of the new words that appeared were nothing more than spelling errors that caught on.

## #7 2010-02-17 22:58:08

phrontister
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### Re: Seeking anwser for a challenging question !!

I did it in

and also in
.

They were the easy ways for me.

Last edited by phrontister (2010-04-01 21:40:06)

"The good news about computers is that they do what you tell them to do. The bad news is that they do what you tell them to do." - Ted Nelson

## #8 2010-04-01 08:21:10

iamfriendly
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### Re: Seeking anwser for a challenging question !!

that's kool , very KOOL Thank you

EYE AM FRIENDLY, THAT'S O U NEED 2 NO,
psst,
Don't trust strangers, EYE AM FRIENDLY, THAT'S O U NEED 2 NO ..........psst, Don't trust strangers......

## #9 2010-04-01 10:17:13

JaneFairfax
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### Re: Seeking anwser for a challenging question !!

#### phrontister wrote:

Do it this way.

A1: 1
A2: 2
B1: -1)^(A1+1)*A1^2

Drag copy handle on cell B1 down to B2. Then select the four cells A1:B2 and drag copy handle down to row 100.

Then enter this:

B101: =SUM(B1:B100)

Last edited by JaneFairfax (2010-04-01 21:50:05)

Q: Who wrote the novels Mrs Dalloway and To the Lighthouse?

## #10 2010-04-01 10:33:51

JaneFairfax
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### Re: Seeking anwser for a challenging question !!

Proof:

If n is even, the sum is equal to

If n is odd, evaluate the sum to n−1 (which is even) and add n2.
­

Last edited by JaneFairfax (2010-04-01 10:42:42)

Q: Who wrote the novels Mrs Dalloway and To the Lighthouse?

## #11 2010-04-01 13:37:50

phrontister
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### Re: Seeking anwser for a challenging question !!

Hi Jane,

#### JaneFairfax wrote:

Do it this way.

A1: 1
A2: 2
B1: = (-1)^(A1+1)*A1^2

Drag copy handle on cell B1 down to B2. Then select the four cells A1:B2 and drag copy handle down to row 100.

Then enter this:

B101: =SUM(B1:B100)

Nice! Much more streamlined than mine...which now looks rather clumsy.

Just a couple of things:

1. More streamlining:
A1: 1
B1: = (-1)^(A1+1)*A1^2
Select these two cells and drag copy handle them down to row 100.
B101: =SUM(B1:B100)

Your formula obviates the need for multiple-row selection.

2. An alternative:
A1: 1
A2: 2
B1: A1^2
B2: -(A2^2)
Select the four cells A1:B2 and drag copy handle them down to row 100.
B101: =SUM(B1:B100)

This alternative utilises the way Excel and OOoCalc paste a 'drag copy' of a multiple-row selection.

Last edited by phrontister (2010-04-02 10:43:55)

"The good news about computers is that they do what you tell them to do. The bad news is that they do what you tell them to do." - Ted Nelson