Mathcast #3: Quick multiplication of two 3-digit numbers
December 9th, 2007 | by Sol |Here’s the You Tube Video that follows up Mathcast #2: Quick multiplication of two 2-digit numbers. The algebra of cross-multiplication explains how this method is derived.
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9 Responses to “Mathcast #3: Quick multiplication of two 3-digit numbers”
By Jonathan on Dec 9, 2007 | Reply
OK, now that changes things. I definitely will practice a bit so that I can keep this as another tool. I can handle at least 4-digit x 4-digit this way, which extends my mental multiplication.
I still look at, for example, 234 x 567 and see 234×7x3×3x3×3. Multiplying by one small digit seems the easiest option for me, when it exists.
From time to time I share ‘tricks’ (techniques) with kids. I’ll let you know, when it comes up, how it goes.
By Sol on Dec 9, 2007 | Reply
Jonathan,
Wow, I’m impressed you can factor numbers so quickly and easily. It’d take me a fair effort to keep track of all those factors of 567.
By Jonathan on Dec 9, 2007 | Reply
The first nine and second nine are not so bad (sum of the digits…), but in this case I saw 560 + 7 first, clearly 7 goes in.
Three by three cross multiplication I think I can handle in my head now… but I still need to see the factors written in front of me.
By Alex Kay on Dec 11, 2007 | Reply
Hey Sol, I *actually* watched the whole video. I usually hate math… But this was really interesting. Thanks for the great content!
Alex
By Sol on Dec 11, 2007 | Reply
@Jonathan - I also need to have the numbers in front of me when I do cross-multiplication but I can do it just like in the video
@Alex - You’re quite welcome. Lots of people think they hate Math! I try to lure them away from that thought.
By program indir webmaster on Jun 30, 2008 | Reply
Idont look is video :S
By Paula on Jul 10, 2008 | Reply
Thank you! I am now really enjoying math!