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# Mental arithmetic; multiplication Watch

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.

Updated: February 16, 2008
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