Objectives
After reviewing this unit you will be able to:
We often use graphs to give us a picture of the relationships between variables. Let's first look at the basic construction of graphs.


A point is the basic relationship displayed on a graph. Each point is defined by a pair of numbers containing two coordinates. A coordinate is one of a set of numbers used to identify the location of a point on a graph. Each point is identified by both an x and a y coordinate. In this unit you will learn how to find both coordinates for any point. You will also learn the correct notation for labeling the coordinates of a point. You will first begin by identifying the xcoordinateof a point.
The xcoordinate of a point is the value that tells you how far from the origin the point is on the horizontal, or xaxis. To find the xcoordinate of a point on a graph:
At the right is a graph with two points, B and D. In this figure:

As we already mentioned, each point is defined by two coordinates, the x and the y coordinate. Now that you know how to find the xcoordinate of a point, you have to be able to find the ycoordinate. The ycoordinate of a point is the value that tells you how far from the origin the point is on the vertical, or yaxis. To find the ycoordinate of a point on a graph:
Looking back at the graph with our points B and D, we now identify the ycoordinate for each.

Once you have the coordinates of a point you can use the ordered pair notation for labeling points. The notation is simple. Points are identified by stating their coordinates in the form of (x, y). Note that the xcoordinate always comes first. For example, in the figure we've been using, we have identified both the x and y coordinate for each of the points B and D.

If a point is lying on an axis, you do not need to draw lines to determine the coordinates of the point. In the figure below, point A lies on the yaxis and point C lies on the xaxis. When a point lies on an axis, one of its coordinates must be zero.


There are times when you are given a point and will need to find its location on a graph. This process is often referred to as plotting a point and uses the same skills as identifying the coordinates of a point on a graph. The process for plotting a point is shown using an example.
Plot the point (200, 300).
Step One  Step Two  Step Three 

First, draw a line extending out from the xaxis at the xcoordinate of the point. In our example, this is at 200.  Then, draw a line extending out from the yaxis at the ycoordinate of the point. In our example, this is at 300.  The point where these two lines intersect is at the point we are plotting, (200, 300). 
You are now ready to try a practice problem. If you have already completed the first practice problem for this unit you may wish to try the additional practice.
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