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**NIH****Member**- Registered: 2005-06-14
- Posts: 33

Take the first 10 natural numbers -- that's 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. Pick any five of the numbers, and write them in a column, in increasing order. Then take the five remaining numbers -- the ones you didn't pick -- and write them in a second column, in decreasing order.

In each row, subtract the smaller number from the larger one, giving you a third column. (See the example below.) Now add the numbers in the third column.

1 9 8

4 7 3

5 6 1

8 3 5

10 2 8

Total: 25

What is your total? Everybody post their total...

2 + 2 = 5, for large values of 2.

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,619

3 10 7

4 9 5

6 5 1

7 2 5

8 1 7

Total: 25

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**Roraborealis****Member**- Registered: 2005-03-17
- Posts: 1,594

1 8 = 7

3 6 = 3

7 5 = 2

9 4 = 5

10 2 = 8

Total: 25!

School is practice for the future. Practice makes perfect. But - nobody's perfect, so why practice?

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**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

I have no imagination at the moment...

1 10 9

2 9 7

3 8 5

4 7 3

5 6 1

--

25

I'm annoyed at this because at a glance it seems easy to prove how it works, but on closer inspection it's not at all!

That or I'm being dumb...

Why did the vector cross the road?

It wanted to be normal.

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**Zach****Member**- Registered: 2005-03-23
- Posts: 2,075

8 - 10 = 2

6 - 9 = 3

4 - 7 = 3

2 - 5 = 3

1 - 3 = 2

----

13

I'm not sure if I got that correct, based on the point of the idea. Perhaps I got it wrong.

Boy let me tell you what:

I bet you didn't know it, but I'm a fiddle player too.

And if you'd care to take a dare, I'll make a bet with you.

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**NIH****Member**- Registered: 2005-06-14
- Posts: 33

Zach wrote:

I'm not sure if I got that correct, based on the point of the idea. Perhaps I got it wrong.

The first column should be in *ascending* order.

mathsyperson wrote:

I'm annoyed at this because at a glance it seems easy to prove how it works, but on closer inspection it's not at all!

I agree; it *looks* trivial, but it's not!

2 + 2 = 5, for large values of 2.

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**Zach****Member**- Registered: 2005-03-23
- Posts: 2,075

Gah. Misread that then.

1 - 10 = 9

2 - 9 = 7

4 - 7 = 3

6 - 5 = 1

8 - 3 = 5

-----

25

Boy let me tell you what:

I bet you didn't know it, but I'm a fiddle player too.

And if you'd care to take a dare, I'll make a bet with you.

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,619

Maths: It is still amazing to me how something that starts so simply (one pebble plus another pebble makes two pebbles) can become so rich and varied so quickly.

I mean, you can make a circle by standing on one foot and dragging the other one around on the sand. Who could possibly think that comparing the distance your feet are apart to the length your foot travels would be a number that has digits going on forever ...

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**NIH****Member**- Registered: 2005-06-14
- Posts: 33

MathsIsFun wrote:

Who could possibly think that comparing the distance your feet are apart to the length your foot travels would be a number that has digits going on forever ...

Indeed. And who could imagine that that ratio is equal to 4 - 4/3 + 4/5 - 4/7 + 4/9 - 4/11 + ... ? Or imagine that it is equal to sqrt(6/1^2 + 6/2^2 + 6/3^2 + ... ) ?

*Last edited by NIH (2005-06-28 07:21:54)*

2 + 2 = 5, for large values of 2.

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,619

Simply inspires awe.

I got into Excel and ran the first one for 50 terms (up to-4/99 ) and it got to 3.1216 (0.02 off)

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**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

You have to go incredibly far to get it accurate though.

I went to the 65536th term (filling an Excel column) and it comes to 3.141577..., meaning it's still only good to 4 decimal places!

I think NIH might have had a typo when telling us the second one...

Why did the vector cross the road?

It wanted to be normal.

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**Mr T****Member**- Registered: 2005-03-30
- Posts: 1,012

i am well confused...

I come back stronger than a powered-up Pac-Man

I bought a large popcorn @ the cinema the other day, it was pretty big...some might even say it was "large

Fatboy Slim is a Legend

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**NIH****Member**- Registered: 2005-06-14
- Posts: 33

mathsyperson wrote:

I think NIH might have had a typo when telling us the second one...

Thanks -- it's fixed now!

Of course, this comes from the well known result, first proved by Euler, that pi^2/6 = 1/1^2 + 1/2^2 + 1/3^2 + ...

2 + 2 = 5, for large values of 2.

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**Roraborealis****Member**- Registered: 2005-03-17
- Posts: 1,594

They were exchanging information about pi 'doccuments'..........

School is practice for the future. Practice makes perfect. But - nobody's perfect, so why practice?

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**Mr T****Member**- Registered: 2005-03-30
- Posts: 1,012

exciting.. -_-

I come back stronger than a powered-up Pac-Man

I bought a large popcorn @ the cinema the other day, it was pretty big...some might even say it was "large

Fatboy Slim is a Legend

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**NIH****Member**- Registered: 2005-06-14
- Posts: 33

Here's a hint as to why the total is always 25.

In the examples above, color or shade (in the first two columns) the numbers 1, 2, 3, 4, and 5. Do you see a pattern?

2 + 2 = 5, for large values of 2.

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,619

Always on unique lines ...

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**TomG****Member**- Registered: 2005-07-18
- Posts: 2

1 10 9

2 9 7

3 8 5

4 7 3

5 6 1

25

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,619

Yep! Still works!

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**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 21,061

1 20 19

2 19 17

3 18 15

4 17 13

5 16 11

6 15 9

7 14 7

8 13 5

9 12 3

10 11 1

That's 100!

Re-arranging,

1 17 16

2 20 18

3 13 10

4 19 15

5 16 11

6 18 12

7 15 8

8 14 6

9 11 2

10 12 2

100 yet again!

Should it be always (n/2)² for 0 to n???

(Just like the side total of a magic square containing numbers 1 to n² is

(n³+n)/2 ??? )

*Last edited by ganesh (2005-07-18 17:03:46)*

It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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**Roraborealis****Member**- Registered: 2005-03-17
- Posts: 1,594

Er.....wasn't it numbers from 1 to 10?

School is practice for the future. Practice makes perfect. But - nobody's perfect, so why practice?

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,619

Ganesh has extended the idea.

It seems logical. Let's try a 6 row one (1 to 12):

1 12 11

3 10 7

4 8 4

5 7 2

9 6 3

11 2 9

Total = 36

That is 6²

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**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

1 2 1

Total=1=(2/2)²

*Last edited by mathsyperson (2005-07-19 02:17:49)*

Why did the vector cross the road?

It wanted to be normal.

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,619

1 4 3

2 3 1

Total = 2²

1 4 3

3 2 1

Total = 2²

1 3 2

4 2 2

Total = 2²

2 4 2

3 1 2

Total = 2²

2 3 1

4 1 3

Total = 2²

3 2 1

4 1 3

Total = 2²

Is that all of them?

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**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

I think so.

-- -- --

Total=0=(0/2)²

*Last edited by mathsyperson (2005-07-19 04:04:42)*

Why did the vector cross the road?

It wanted to be normal.

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