Tuesday, December 1, 2009
Math trick for learning the nines times tables
My older son is learning his times tables. For me, this brings back painful memories of memorizing columns of data. Fortunately, Kerm has Itinerant Cryptographer for a father. My husband is something of a math junkie. He taught Kerm a little trick for multiplying by nines. It works for 9x1 through 9x10:
* The first digit of the answer is always one less than the number you are multiplying by,
* the digits of the answer have to add up to nine.
So, let's say that you are multiplying 9 x 3. The first digit of the answer starts with a "2". And 9 minus 2 is 7, so the answer is 27.
9 x 1? The first digit would be zero. Nine minus zero equals nine, so the answer is 9.
Isn't that neat? (Where was Itinerant Cryptographer when *I* was learning my times tables?
Photo credit: Paul Joseph, through a Creative Commons Attribution 2.0 License via Flickr.