AI Tutor
Imagine shining a light straight down and up onto a graphed line so it casts a shadow directly onto the horizontal xx-axis. That shadow tells you everywhere the graph exists from left to right.
The domain of a function is the set of all possible input values (xx-values) shown on the graph.
What happens when a straight line has arrows on both ends and keeps going forever?

Continuous Lines with Arrows

When a linear graph has arrows on both ends, it never stops traveling left or right. Even if a line looks steep, it still widens horizontally without end.
Because the graph covers every single xx-value, its domain is all real numbers, written in inequality notation as −∞<x<∞-\infty < x < \infty or in interval notation as (−∞,∞)(-\infty, \infty).
What if the graph stops at specific points instead of going on forever?

Line Segments and Endpoints

A solid circle (closed dot) means the endpoint is included in the domain (using ≤\le or ≥\ge). An open circle (hollow dot) means the graph gets infinitely close to that point, but does not include it (using << or >>).
To find the domain of a segment, look only at the horizontal boundaries: find the leftmost xx-value and the rightmost xx-value.