You are Here: Home >< Maths

# Mental arithmetic; multiplication

Announcements Posted on
Would YOU be put off a uni with a high crime rate? First 50 to have their say get a £5 Amazon voucher! 27-10-2016
1. Hey everyone, kinda random but I was wondering what the easiest way of doing multiplication of three digit numbers in your ead would be, ie 234 x 127.

This is just out of boredom at work and I began to think aren't I cool... but anyway!

Two digit numbers I can cope with, either by doing that crummy box method:
like 12 x 22
x 20 2
10-200 20
2- 40 4

An adding. Or by doing (22 x 10) + (22 x 2).

It's a bit more tricky with three digit numbers using the box as there is nine boxes now so nine numbers to remember. I can just about to do it.

But is there an easier way?

[I've already explained it's just a wandering thought and It's not like I do this in my spare time so no poking fun lol]

Thanks
2. The "box" method should work OK. Have you not used the traditional put-one-number-on-top-of-the-other way?

For two digit numbers, if they're both even or both odd, I like to use difference of two squares, eg.

18 x 8 = (13 + 5)(13 - 5) = 13^2 - 5^2 = 169 - 25 = 144

even though it's pointless and confusing.
3. thats a really nice method with difference of 2 squares!

does anyone have a mental routine sort of algorithm for working out the square root of a number?
4. (Original post by Totally Tom)
thats a really nice method with difference of 2 squares!

does anyone have a mental routine sort of algorithm for working out the square root of a number?
Yes: Start-->Find calculator--->*Calculator found?--->yes--->use calculator--->end

*calculator found?--->no--->give up-->end

Also, if i'm multiplying 3 digit numbers i usually just do the whole multiply + add thing, but i look for a calculator or pen/paper first.
5. (Original post by Totally Tom)
thats a really nice method with difference of 2 squares!

does anyone have a mental routine sort of algorithm for working out the square root of a number?
You can do long square rooting by hand, but it is so pointless:

http://www.bbc.co.uk/dna/h2g2/A827453
6. didn't understand TSRs form
7. (Original post by DeuceSevenOff)
You can do long square rooting by hand, but it is so pointless:

http://www.bbc.co.uk/dna/h2g2/A827453
In some respects, it is somewhat easier to apply that algorithm when you have converted the numbers to binary first.
8. (Original post by Totally Tom)
thats a really nice method with difference of 2 squares!
Thanks, been using it for years, before I even knew what difference of two squares was.

does anyone have a mental routine sort of algorithm for working out the square root of a number?
I'd probably go for binomial expansion.

(Original post by malsi101)
13^2 wht does tht mean?!?!?!
Just a way of writing 13 squared, i.e. 13 x 13.

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: February 16, 2008
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

Find out here

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read here first

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams