## Thursday, February 16, 2006

### HOWTO: Is an integer divisible by 3?

Here's another mathematical shortcut. If you didn't already know it, I hope it saves you some time!

2. if there is more than one digit in the sum and you are not sure if the sum is divisible by 3, just add the digits together again. This converges quickly to a number that is obviously divisible or not divisible by 3.
3. If the resultant sum is divisible by 3, the original integer is divisible by 3. If it isn't, neither is the original.

So there you have it.

Burton MacKenzie
http://www.burtonmackenzie.com

harmzie said...

I actually knew that one! Yay school system! Or at least one lone useful teacher in the lot. Sorry, I don't have a reference to prove that I knew it before (your conditions for commenting).

That was way too much trouble (get an account, bla bla bla) for such an inane comment, so I'll add:

FISH STICKS!!! I have no proof of that either.

harmzie said...

Stumbled on this tonight. Freaked me out.

http://www.hunkinsexperiments.com/pages/multiplication.htm

[Freaky Russian peasants]