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## The Magic of 9

Published on Thursday, September 10, 2009 in , , ,

Since yesterday was 9/9/09, it seems like the perfect time to look at the mysteries of 9.

BTW, the actor who was singing about 9s in yesterday's video, and who apparently learned about 9s from Fleet the Genii, is Reg E. Cathey, whom you may know from a surprising number of places.

We'll start off with something simple. The following video teaches an easy way to learn the 9 times table from 1 up to 10. It's so straightforward, there's no audio required to help you understand it:

Probably one of the most famous 9 tricks would have to be the classic 1089 trick. The explanation is surprisingly simple, despite the large numbers involved.

Why is 9 so surprising? Sol from Wild About Math took a look at this phenomenon last year, and explains it pretty well. The best part of this article is the mathematical magic tricks you learn while trying to understand the principles being taught. Yes, the 1089 trick shows up here, as well.

Sol's article briefly mentions “casting out nines”, which I remember learning about long ago. It's a great way to check your math on an arithmetic problem, but the teaching of it seems to have fallen out of favor decades ago. The usefulness and simplicity of it, once learned, is so elegant, that there are people today who, when they finally learn about casting out 9s, become enraged that they were never taught it!

Yes, 9s can simplify magic tricks, but math as well. Sophie B. shows us how to use 9's amazing properties to multiply by 9, 99, 999 and even 19! I not only like the methods taught here, but the amusing way the story is used to present it.

Even if you missed 9/9/09 itself, you still have the rest of the month to improve and show off your 9 prowess in 9/09!

### 2 Response to The Magic of 9

Jay
6:55 PM

Hey, Jay here, long time Grey Matters. I am crazy about mental calculation and noticed an interesting thing. You can cast out 99's just like 9's by adding up pairs of numbers. This has teh added bonus of catching place values errors. With casting out nine's it will not guard against errors such as getting 789 instead of 7890. The nines test doesn't guard against place value shifts. The nine's test catches 89% of errors whereas the 99's test catches 99% of errors! Just thought I'd share

Anonymous
2:21 AM

This post refers to a webpage at themathlab.com which teaches "casting out nines."

The internet is great for spreading misinformation. If you link to another webpage, you should be responsible for pointing out errors on that webpage.

And the folks at themathlab.com should held responsible for their careless reading of the Asimov book cited on their page. Asimov does not misuse the word "augend," but themathlab.com article does. (See link at end of third from last paragraph.)

If one is going to show off by using obscure words, one ought to make sure he/she is using the word correctly!