Cool math shortcuts

[kyro]

I do this kinda crap when I'm bored. The one Homer came up with is something I figured out when I was 13 or so. There's a bunch that I read in some book, like squaring numbers where the first two digits are the same and the second two digits add up to ten.

39 * 31
(3*(3+1))(9*1)
(12)(09)
1209