## College Algebra (10th Edition)

A half-closed (or half-open) interval has one of the endpoints included, and the other excluded. For example, $(2,5]$ is an interval, $2 \lt x \leq 5$ that contains 2.1,2.0001,2.000000001... but not 2 itself. It is open from the left, and closed from the right, as it contains the number 5. So, you don't need $\pm\infty$ to be endpoints of half-closed intervals.