Guilloché pattern and other Web apps

Here is nice Guilloché pattern generator done as web app. These patterns have long history and were used as ornaments by crafters. Wikipedia article about Guilloché is here.

The site has plenty interesting information about geometry, patterns, etc. For exampe a Butterfly curves web application.

Multiplication with your fingers

Here is interesting trick to multiply numbers in second half of multiplcation table (6-10). I do not recall where I learned it from but it has been around for quite some time. Best way to explain how it works is on example:

to multiply 6 by 7 you open 1 finger on one hand (6-5 = 1) and 2 fingers on another (7-5 = 2), then you multiply number of closed fingers on your hands and add it to sum of open fingers multiplied by 10. In our case -

4 (closed fingers on one hand) x 3 (closed fingers on another hand) + (1 + 2) * 10 = 42

another example:

8*9 translates to : 2*1 + (3+4) * 10 = 72

:)