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## #1 2007-08-09 22:30:52

MathsIsFun
Registered: 2005-01-21
Posts: 7,685

### Subtraction

What do you think of this basic page on Subtraction ?

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

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## #2 2007-08-10 00:03:54

luca-deltodesco
Member
Registered: 2006-05-05
Posts: 1,470

### Re: Subtraction

any change of getting my method up there? i know other people have found it useful before:

basicly its the same as the borrowing but you think about it in a different way. rather than changing the numbers and then doing in the example below for first step 12-8, you do 8-2 = 6, which from 10 gives 4, then like addition, put a carry 1 above the line to be added onto the next number for subtraction.

i think the reason i do it like this, is because even now it takes me a little while to think about something as simple as 12-8 or 16-9. whereas doing 8-2 and 9-6 is instant, and then subtracting that from 10 becomes automatic aswell without thinking.

i dunno, i find it quicker and easier to work with than borrowing where you have to start crossing numbers out and writing more numbers etc.

heres an animation of it: (0.5fps lol)
http://denvish.net/ulf/100807/46955_sub.php

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## #3 2007-08-10 00:38:30

Patrick
Real Member
Registered: 2006-02-24
Posts: 1,005

### Re: Subtraction

In school I learned the method shown on the site, but I've always used the method explained by luca..

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## #4 2007-08-10 00:59:07

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

### Re: Subtraction

I prefer luca's method as well. The main problem with the one taught in schools is that it can need "double carrying".

For example, 304 - 27.
You need to carry a ten to make the 4 into a 14, but there aren't any tens so you need to overflow into hundreds. The 0 then becomes a 10 which quickly then becomes a 9, and you end up with a bunch of confusing scribbles on the page and it's easy to get mixed up.

Why did the vector cross the road?
It wanted to be normal.

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## #5 2007-08-10 11:58:20

MathsIsFun
Registered: 2005-01-21
Posts: 7,685

### Re: Subtraction

Cool. I will try to make a page on it, and you guys can then correct it.

Another method:

``````  32
- 15
---``````

Take tens complement of 15: 85

``````  32
+ 85
---
117``````

Drop the "overflow" 1 to get the answer: 17

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

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## #6 2007-08-10 22:39:18

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

### Re: Subtraction

Ooh, that's quite neat. Probably not the best 'core' method to use, but a nifty shortcut if you're subtracting something like 95.

Why did the vector cross the road?
It wanted to be normal.

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## #7 2007-08-11 13:37:25

MathsIsFun
Registered: 2005-01-21
Posts: 7,685

### Re: Subtraction

OK, here is a draft of what I am calling Quick Subtraction

I think another example would help, but don't want to make it until I have feedback.

If you can re-word the description to something that is easier to understand that would be good.

This method may actually have a correct name, if anyone knows it let me know.

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

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## #8 2007-08-11 22:36:37

Identity
Member
Registered: 2007-04-18
Posts: 934

### Re: Subtraction

I find using Quick Subtraction is just as fast as normal.

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## #9 2007-08-12 03:28:20

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

### Re: Subtraction

3. Add 1 to the digit on the left

Why did the vector cross the road?
It wanted to be normal.

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## #10 2007-08-12 12:17:03

MathsIsFun
Registered: 2005-01-21
Posts: 7,685

### Re: Subtraction

Oh, yeah

Anything else?

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

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## #11 2007-08-12 14:29:23

John E. Franklin
Member
Registered: 2005-08-29
Posts: 3,588

### Re: Subtraction

"2 is less than 6", not "2 less than 6" in sketch-vid.

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## #12 2007-08-12 15:39:11

MathsIsFun
Registered: 2005-01-21
Posts: 7,685

### Re: Subtraction

Hmmm ... yes, could be confusing otherwise.

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

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## #13 2007-08-14 19:25:00

MathsIsFun
Registered: 2005-01-21
Posts: 7,685

### Re: Subtraction

I have been hard at work, and now have three methods described.

I have linked to them from the website, so if you find an error, let me know quick!

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

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