Stupid math tricks

[Doggett]

You have to add each single digit. 345 IS evenly divisible, and 3 + 4 + 5 = 12, also evenly divisible. You're trying to divide the wrong number,

Oh, I see...

[Derf]

Multiplying on your fingers:

By nines:
http://www.youtube.com/watch?v=oamy8L2lDZM&feature=related


And by 6,7,8,9, or 10:
http://www.youtube.com/watch?v=_eFnzNw10Qc&feature=related

[ghouck]

That's nothing... here's something really weird, the mystic power of the number 1089.

Take any three digit number, each with a different digit, highest digit first ex 731.  Subtract from that the inverse of the number (137), you take 731-137, you get 594.  Now add the inverse of that number (594 + 495) and you get 1089.

Try it, it works with about any three digit number.

But, let's say you don't want to put the highest digit first, so you do 154 - 451, you get -297 (negative 297).  You add -297 to 792, you get 495.  Now, add 495 to 594, and what do you get?

1089!

It's freaking creepy!!!

Well, let's say you don't want the digits to all be different, so let's take 855 minus 558, you get 297.  297 plus 792 is, you guess it, 1089!

Now, watch this... 998 - 899, you get 99.  But, add 99 to 99, you get 198.  Add 198 to 891, and once, again, 1089.

I don't know what it all means, but it kind of scares me...

I was just looking at this and I see why it works. When you take a number, reverse it and subtract that from the original number, you ALWAYS get a number with a 9 in the middle and the first and last digits always add up to 9. So when IT is flipped and added, the two first digits always equal 9, the two third digits always equal 9, and the two middle digits always equal 18.