Thursday, August 14, 2014

Math Trick: Multiplication by 9

This popular trick is a fun one to show children who are learning their times tables:


Some viewers have commented that this is not "the way" to teach children math. Perhaps not, but I ask, how is this any worse that teaching them their times tables by rote memorization? Why not teach them what it actually means to multiply one number by another? When I was 6 years old, my dad drew a picture for me to explain multiplication. The picture looked something like this:


He said to calculate \(3 \times 5\) all I had to do was think of three rows of five squares each and add up the squares. In other words I just had calculate \(5 + 5 + 5 \). Using this method, I could do small multiplication problems in my head before I was ever forced to memorize them.

Having said all this, my purpose in making the preceding video is to provide a fun way reinforce the learning of multiplication tables. It is not meant to be used as a primary means of multiplication.

For those with a bit of mathematical maturity, an interesting question to ask oneself is: why does this trick work?

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