Mathcast #2: Quick multiplication of two 2-digit numbers
December 1st, 2007 | by Sol |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.
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Enjoy.
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19 Responses to “Mathcast #2: Quick multiplication of two 2-digit numbers”
By Jonathan on Dec 1, 2007 | Reply
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
= 62×7x3×2
= 434×3x2
= 1302×2
= 2604
Now, this is not generally applicable, but it would be structurally different.
By Sol on Dec 1, 2007 | Reply
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.
By Jonathan on Dec 1, 2007 | Reply
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!
By Karen (Karooch from Scraps of Mind) on Dec 3, 2007 | Reply
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.
By Sol on Dec 3, 2007 | Reply
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
By Kalid on Dec 3, 2007 | Reply
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.
By Sol on Dec 5, 2007 | Reply
Kalid,
Yes, not needing a scratchpad is nice. Wait till I show multiplication for a pair of 3-digit numbers.
By Kalid on Dec 5, 2007 | Reply
Great, I’m looking forward to it!
By ABHINANDH G on Mar 9, 2008 | Reply
NICE THANK U FOR GIVING SUCH AN ENCHANTING EXPERIENCE …I AM VERY MUCH SATISFIED KNOWING THESE MATH TRICKS.. AND EXPECTING MORE ..THANK U
By ABHINANDH G on Mar 9, 2008 | Reply
IS THERE SOME THING WHICH CAN BE APPLIED IN ENGG.SOMEWHAT GOEMRTRY OR SIMILAR ONES
By Abdul shakur on Apr 16, 2008 | Reply
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.
By shishir on Feb 28, 2009 | Reply
Hey!
Daz famous Vedic mathematics way for calculation
By julie on Jun 26, 2010 | Reply
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
By Teresa on Aug 17, 2010 | Reply
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.