I'm looking for someone to point me in the right direction. I can learn things for myself, but I don't know where to begin.

What does one even call that? Is it part of Calculus? Right now I'm studying PreCal in high school, so I really have no idea what calculus consists of. We've only done stuff that I would consider algebra and geometry/trig thus far. That one example is one of many that I'm interested in... but to me, they all seem almost the same. It reminds me of something we did last year in Algebra II, but I don't think it's quite the same (geometric/arithmetic sequences).