Are arithmetic sequences and series even tested on the ACT? I'm using Barron's 36 and I have never even seen this stuff before. Should I bother to remember it?
Arithmetic sequences are pretty rare on the ACT - you'll see a question (never more than one) asking about them once every 2-3 tests. As we advise in our blog article on sequences in the ACT Math section, you should only study sequences once you feel you have a good grasp of the other more common ACT Math topics like angles, integers, ratios, slopes, and triangles.
Basically, if you're already good on all of those topics and are aiming for a perfect score on the math section, make sure you know how to answer sequence questions. Otherwise, focus your attention on areas that'll net you more points.