@druinok sent me a formula the other day on Twitter that predicted the amount of time between insurgent attacks in battle. She asked me when I thought we'd be able to use this in Algebra 2. I thought that it looked more like a precalculus application (when dealing with sequences) and may show it then, but it also got me thinking that I'd like to find some formulas that people actually use outside the classroom.
I did a little bit of research online and this is what I came up with. If you have any more, please pass them on! :)