Zyniker's Weblog

For those of you out there who, like me, find the unique, pattern-forming properties of 9s when adding, multiplying, and dividing to be not only interesting but also entertaining, the following may pique your interest:

Do Speedy Math in Your Head – Wired How-To Wiki:

To square a number like 14, identify the preceding round number — in this case, 10. Since you subtracted 4 to get 10, add 4 to 14 to get 18 and multiply that by 10 (*the round number). Add to that the square of 4: 180 + 16 = 196.
To find the square of the next number in a series without multiplying: Say you know the square of 12 is 144. Add 12 to 144 and add the next number — in this case, 13 — to find the square of 13: 144 + 12 = 156 + 13 = 169, the square of 13; and then, 169 + 13 = 182 + 14 = 196, etc.
11 times any two-digiter

To multiply, say, 11 x 32, add the digits of 32 (3 + 2 = 5) and insert the sum between them: 352. Numbers with two-digit sums use a slight variation: For 11 x 84 (8 + 4 = 12), add the 1 from 12 to the 8 and leave the 2 in the middle: 924.
Magic number

Ask a fan to think of any number. Then have them double it, add 12, divide by 2, and subtract the original number. Before they’re done, tell them the answer: 6. It will always be 6.