How to Define the Domain of a Graph

By Jessica Reed

Algebra classes have you stressed out? Whether it's a high school algebra class or you are preparing for a college algebra class, math can be confusing for many students. Here is a description of how to find the domain of a graph. Domain describes the length of the X-axis that the points of the graph, or a line, cover.

Find the farthest number to the left on the X-axis of the graph. The X-axis is the axis that runs left to right on the graph, horizontally. The number farthest to the left is the first number you need to write the domain. If the graph has an arrow on the left side of the line you are finding the domain of, then that means it goes on for infinity.

Find the farthest number to the right side of the graph. You will still be using the X-axis, the horizontal one. If there is an arrow pointing to the right, this means that the line goes on forever in the right direction. Thus, infinity will be how far the right side of the graph stretches. Otherwise, there will be one number that is farthest to the right, and no other points, or points on the line, will go passed it.

Take the number farthest to the left, and farthest to the right and make an ordered pair out of them. To do this, draw a parenthesis. Write the number that you found to be farthest left, first. If it goes on for infinity draw the infinity symbol. The infinity symbol looks like the number 8 lying on its side. Write a comma after that number and write the number you found to be the farthest to the right, or the infinity symbol. Close the parentheses, and you have found the domain of your graph.