Teaching Math and Music with Technology

I have come across some fun math problems (yes, math CAN be fun!) through the internet and friends.  Here is a video of Japanese Multiplication.  First watch the video, and then see if you can figure out how they are multiplying  before you read my explanation below!

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Well, did you figure it out?  Let’s take a closer look at the first example:

Now take it apart:

• The first set of lines, the green ones in my image above, represent 2 sets of 10 , or 20
• The second region with one line, which is orange in my image, represents 1 set of 1
• Together these sets of lines, read top to bottom,  represent 2×10 + 1×1  which is expanded notation for 21

Let’s look at the next set of lines that were drawn perpendicular to these lines:

• The first region that contains the 1 blue line represents 1 set of 10 or 1×10
• The second set of lines that were drawn, the three red lines, represent 3 sets of 1
• Together these 4 lines, read left to right, represent 1×10+3×1 = 13 in expanded notation.

Now for the tricky part!

Those of you who have ever FOIL-ed in Algebra will recognize the process of distributing the values by “First, Outer, Inner, Last”

Here is a quick Algebra example to remind you
(x+3)(2x+5) =

First = x * 2x = 2x^2
Outer= x*5 = 5x
Inner = 3*2x = 6x
Last = 3*5 = 15

Then,  2x^2 + 5x +6x +15 = 2x^2 + 11x +15 (the Outer and Inner were “like” terms, so could be added together)

Now back to the arithmetic.   If you look at the product 21×13 by separating out each factor by its place values, you have:

(20 + 1)(10 + 3)  and now you can FOIL out the values, just like in the Algebra problem!

First = 20×10 = 200
Outer =  20×3 = 60
Inner = 1×10 = 10
Last =  1×3 = 3

The 200 is represented by the 2 sets of crossing lines circled in yellow on the image above- that location on the paper represents the hundreds place value, so having a 2 in the hundreds location represents 2×100 = 200.  In the video a 2 is placed as the first digit of the product, which will be the hundreds place.

Next:

The 60 is represented by the 6 sets of crossing lines in green on the top right
The 10 is represented by the 1 set of crossing lines in green on the bottom left

Together the 60+10 gives 70.  In the video, the areas circled in green on the image above both represent the tens place value, so they are adding up the 6 crossed marks and the 1 crossed mark to get 7 sets in the tens place, or 7×10=70.    They then place a 7 to the right of the 2 in the product (placing it in the tens place)

Finally:

The 3 is represented by the 3 crossed marks in the lower right (circled in red on the image above).   This area of the paper represents the ones place, so we have 3×1 = 3.   They then place a 3 to the right of the 7 in the product, placing the 3 in the ones place.

This gives the final product of 200+60+10+3 or 200+70+3 = 273

~Now look at the second product in the video and see if you can figure out how it works!