One of the things that was used less and less time was spent on, but they're convergent sum of a series that eventually approximates some function's value with

One of the things that was used less and less time was spent on, but they're convergent sum of a series that eventually approximates some function's value with increasing accuracy, from what I remember. You can get a sine value from one. Actually proving them, I can't remember how, but the class treated it pretty much as a short bonus topic.