Wild About Math! Making Math fun and accessible

1Dec/0715

Mathcast #2: Quick multiplication of two 2-digit numbers

I've produced a second mathcast, this one using Camtasia screen recorder. I've posted the mathcast on You Tube and you can watch it right here without leaving the blog. This video shows you how to quickly multiply together a pair of two digit numbers without having to write down the intermediate results.

This is the second in a series of videos I'll be producing on arithmetic tips and tricks.

The first mathcast was here. It was about very quick multiplication for special cases of numbers.

If you enjoy this video check out all of the Wild About Math! mathcasts.

Check back frequently for new videos and for interesting Math-related articles, especially ones with fun activities. Tell your friends and subscribe to the RSS feed or email list to stay current on Wild About Math! happenings.

Enjoy.

Comments (15) Trackbacks (5)
  1. I don’t appreciate a great difference from the standard algorithm.

    . ab
    x cd

    Standard: dxb (carry) dxa (add), cxb (carry), cxa (add), add 2digit+digit.
    Cross: dxb (carry), dxa, cxb, add carry+2digit+digit (carry), axc (add)

    They look like the same steps, with the order modified.

    Compare at:

    . 62
    x 42

    = 62x7x3x2
    = 434x3x2
    = 1302×2
    = 2604

    Now, this is not generally applicable, but it would be structurally different.

  2. Hi Jonathan,

    The benefit of this approach is in not having to write down the partial products. More arithmetic can be done in one’s head. When I do the video on multiplying 3 digit numbers together I think the advantage will be more apparent than in 2×2 multiplication.

  3. OK, I look forward. At this point it’s been so long since I’ve written down 2×2 that it may just not take. 3×3 though, I might enjoy that!

  4. My first reaction was that this is no quicker than the standard method and may even be a little slower. But then I realised that it’s a method that you can do in your head whilst the ‘normal’ method usually requires writing down. So I can see the application.

    But I’m not ready to throw away my calculator yet Sol.

  5. Karen,

    I’ll soon be making another video showing multiplication of a pair of 3-digit numbers. Then this approach will make more sense.

    If you learn to use an abacus really well you might be willing to throw your calculator away :)

    Sol

  6. Hi Sol, nice video. I really like the ability to multiply without needing a scratchpad; the “standard” method isn’t that useful because it’s slow enough where you might as well just open up your calculator.

    This reminds me of a trick I learned once: when adding numbers, go from right-to-left and you avoid carries. This approach seems to be similar in that regard.

  7. Kalid,

    Yes, not needing a scratchpad is nice. Wait till I show multiplication for a pair of 3-digit numbers.

  8. Great, I’m looking forward to it! :)

  9. NICE THANK U FOR GIVING SUCH AN ENCHANTING EXPERIENCE …I AM VERY MUCH SATISFIED KNOWING THESE MATH TRICKS.. AND EXPECTING MORE ..THANK U

  10. IS THERE SOME THING WHICH CAN BE APPLIED IN ENGG.SOMEWHAT GOEMRTRY OR SIMILAR ONES

  11. Waw,
    It’s a lovely way of doing quick maths especially for the kids who have long lost their interest in the subject.Please do bring more of such.

    Thanks for the intiative and God bless.Its a good charity to the world.

  12. Hey!
    Daz famous Vedic mathematics way for calculation

  13. I really appreciate…additional knowledge for today!..am HM student in University.Now thanks,i go on my assignment well.I have so fun in our math121a.I love it,ty:)…julie

  14. This was GREAT! I have feared math all my life! This was fun and so easy to learn! I am 55 yrs old and I want to thank you for this learning tool.

  15. thank you for using your precious time to teach the people.Bye


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