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Mental arithmetic; multiplication

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    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 :cool: 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
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    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.
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    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?
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    (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.
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    (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
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    didn't understand TSRs form
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    (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.
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    (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.

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Updated: February 16, 2008
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