Parentheses exclude the endpoint. Brackets include it. (2, 5] is x > 2 and x ≤ 5. Inequalities as a set are in inequalities.
Open versus closed
Infinity always gets a parenthesis. Zero-product roots are the usual endpoints, in the zero-product property. Domain and range often want this notation, in domain and range.
The bottom line
( means out. [ means in. Infinity is always open.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.