Finding Intercepts on a Graph
When we talk about a line's intercepts, we most often mean the x-intercept and the y-intercept. These are locations where the line intercepts or crosses the x- or y- axis. A more formal definition is as follows:
x-intercept: where a line or curve intersects the x-axis, at y = 0
y-intercept: where a line or curve intersects the y-axis, at x = 0
Below is the graph of a line, with both intercepts shown:
Notice that the x-intercept always has a y-coordinate of 0, and the y-intercept always has an x-coordinate of 0.
Determining Intercepts from a Table
Keeping the above note in mind, we can easily identify intercepts from a table of x and y values when we notice that one of the coordinates is zero! Take a look:
We see that the second pair (0, 12) contains a zero. In this coordinate pair, the x-coordinate is zero, which means this represents a y-intercept. The y-intercept of the line is at the point (0, 12). We also notice a zero in the coordinate pair (4, 0). In this case, the y-coordinate is zero, so this is the x-intercept. The line crosses the x-axis at the point (4, 0).
Determining Intercepts from an Equation
How can we use the equation of a line to find its x– and y– intercepts? A common form of a line is in slope-intercept form: y = mx+b. This form is called slope-intercept form, because b represents the y-coordinate of the y-intercept (and we know that its x-coordinate is 0). We can also use the equation in this form to find the x-intercept. Since x-intercepts have a y-coordinate of 0, we set the equation equal to zero and solve for x. This is shown in the example below:
the location on a graph where a line or curve intersects the x-axis: (x, 0)
the location on a graph where a line or curve intersects the y-axis: (0, y)