Geometric Multiplication Method

I’ve seen many “alternate” methods of adding, subtracting, multiplying, and dividing, but this one is new to me (and in the opinion of this math-nerd author, very slick):

Youtube link

(For my students) Why does this work?

(For anyone) Are there other geometrical math tricks like this one?

3 Responses

  1. Ugh. Try 78 x 69. Let me know if you would have counted the right number of intersections if you didn’t already know the answer.


  2. I would never suggest this as “the method” to teach students — it’s simply the actual method that was used by the Mayans, and worth an exploration activity.

    It does get, as you point out, wildly impractical, and it’d be good to ask students why.

    Another totally impractical method that’s still fun to look at is the multiply-by-2-divide-by-2 algorithm for multiplying (I’ve also heard it called the Russian Peasant Algorithm). If you have a student that can explain why *that* one works without any help, I think it’s safe to say they know their arithmetic.

  3. […] This method from YouTube, which Jason Dyer of The Number Warrior pointed out in this post and in the comments here.    It’s like a visual depiction of grid multiplication.  […]

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