Hey there Sign in to join this conversationNew here? Join for free

Mental arithmetic; multiplication

Announcements Posted on
    • Thread Starter
    • 0 followers
    Offline

    ReputationRep:
    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
    • 7 followers
    Offline

    ReputationRep:
    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.
    • 1 follower
    Offline

    ReputationRep:
    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?
    • 0 followers
    Offline

    ReputationRep:
    (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.
    • 0 followers
    Offline

    ReputationRep:
    (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
    • 8 followers
    Offline

    ReputationRep:
    didn't understand TSRs form
    • 13 followers
    Offline

    ReputationRep:
    (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.
    • 7 followers
    Offline

    ReputationRep:
    (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.

Reply

Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?

    this is what you'll be called on TSR

  2. this can't be left blank
    this email is already registered. Forgotten your password?

    never shared and never spammed

  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
    your full birthday is required
  1. By joining you agree to our Ts and Cs, privacy policy and site rules

  2. Slide the button to the right to create your account

    Slide to join now Processing…

Updated: February 16, 2008
New on TSR

Which unis are the poshest?

See what students perceive to be the classiest establishments in Britain

Article updates
Reputation gems:
You get these gems as you gain rep from other members for making good contributions and giving helpful advice.