Discussion about math, puzzles, games and fun. Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ π -¹ ² ³ °

You are not logged in.

- Topics: Active | Unanswered

Pages: **1**

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,608

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

Offline

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

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

The Beginning Of All Things To End.

The End Of All Things To Come.

Offline

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

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

Support MathsIsFun.com by clicking on the banners.

What music do I listen to? Clicky click

Offline

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

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.

Offline

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,608

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

Do addition:

```
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

Offline

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

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.

Offline

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,608

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

Offline

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

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

Offline

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

3. Add 1 to the digit on the

left

Why did the vector cross the road?

It wanted to be normal.

Offline

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,608

Oh, yeah

Anything else?

Offline

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

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

**igloo** **myrtilles** **fourmis**

Offline

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,608

Hmmm ... yes, could be confusing otherwise.

Offline

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,608

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

Main page on Subtraction

Subtraction by Regrouping (Borrowing)

Quick Subtraction

Subtraction by Addition (Complements Method)

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

Offline

Pages: **1**