Subtraction

MathsIsFun · 2007-08-09 22:30:52

What do you think of this basic page on Subtraction ?

luca-deltodesco · 2007-08-10 00:03:54

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

Patrick · 2007-08-10 00:38:30

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

mathsyperson · 2007-08-10 00:59:07

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.

MathsIsFun · 2007-08-10 11:58:20

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

mathsyperson · 2007-08-10 22:39:18

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

MathsIsFun · 2007-08-11 13:37:25

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.

Identity · 2007-08-11 22:36:37

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

mathsyperson · 2007-08-12 03:28:20

3. Add 1 to the digit on the left

MathsIsFun · 2007-08-12 12:17:03

Oh, yeah

Anything else?

John E. Franklin · 2007-08-12 14:29:23

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

MathsIsFun · 2007-08-12 15:39:11

Hmmm ... yes, could be confusing otherwise.

MathsIsFun · 2007-08-14 19:25:00

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!

Math Is Fun Forum

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

Subtraction

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

Re: Subtraction

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

Re: Subtraction

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

Re: Subtraction

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

Re: Subtraction

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

Re: Subtraction

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

Re: Subtraction

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

Re: Subtraction

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

Re: Subtraction

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

Re: Subtraction

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

Re: Subtraction

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

Re: Subtraction

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

Re: Subtraction

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