Graphing Linear Inequalities
Understanding Intercepts
Meet the Intercepts
Every straight line graphed on a coordinate plane has to cross the axes somewhere, unless it's perfectly horizontal or vertical and runs parallel to an axis. The points where the line crosses the x-axis and the y-axis are special. They're called the intercepts.
Think of the x and y axes as two main roads that cross. Any other road that isn't parallel to them will eventually intersect both. The intercepts are simply those intersection points. They are key landmarks for understanding the position of our line.
y-intercept
noun
The point where the graph of a line crosses the vertical y-axis. At this point, the x-coordinate is always 0.
x-intercept
noun
The point where the graph of a line crosses the horizontal x-axis. At this point, the y-coordinate is always 0.
Finding Intercepts
Finding the intercepts is a straightforward process. It all comes down to one simple rule for each:
- To find the y-intercept, set and solve for .
- To find the x-intercept, set and solve for .
This method works for any form of a linear equation. Let's see how it applies to the two most common forms.
Finding an intercept is just a matter of setting the other variable to zero.
From Slope-Intercept Form:
The slope-intercept form actually gives you one intercept for free. The '' value is the y-intercept. Why? Let's test our rule by setting .
So, the y-intercept is always at the point .
Let's find both intercepts for the equation .
Y-intercept: We can see that , so the y-intercept is .
X-intercept: Now, we set and solve for .
The x-intercept is at the point .
From Standard Form:
The process for standard form is the same, but it requires a little algebra for both intercepts. Let's use the equation as an example.
Y-intercept: First, set .
The y-intercept is .
X-intercept: Next, set .
The x-intercept is . Knowing these two points is enough to graph the entire line.
Why Intercepts Matter
Intercepts aren't just abstract points on a graph. They often represent meaningful starting points or break-even values in real-world scenarios.
For example, if a graph shows the amount of money in your savings account over time, the y-intercept is the initial amount of money you started with (at time zero). If a graph shows the distance remaining on a road trip, the x-intercept is the point in time when you arrive at your destination (distance remaining is zero).
Finding the intercepts is a fundamental skill for understanding and graphing linear equations. It gives you two solid points of reference that anchor your line in the coordinate plane.
To find the x-intercept of a line from its equation, what value must you substitute for y?
What are the x- and y-intercepts of the linear equation ?
