4.1: The Coordinate Plane
In Lesson 1.6, you graphed ordered pairs. This lesson will expand upon your knowledge of graphing ordered pairs to include vocabulary and naming of specific items related to ordered pairs.
An ordered pair is also called a coordinate. The \begin{align*}y\end{align*}
A twodimensional (2D) coordinate has the form \begin{align*}(x,y)\end{align*}
The 2D plane that is used to graph coordinates or equations is called a Cartesian plane or a coordinate plane. This 2D plane is named after its creator, Rene Descartes. The Cartesian plane is separated into four quadrants by two axes. The horizontal axis is called the \begin{align*}x\end{align*}
The first value of the ordered pair is the \begin{align*}x\end{align*}
Multimedia Link: For more information on the Cartesian plane and how to graph ordered pairs, visit Purple Math’s  http://www.purplemath.com/modules/plane.htm  website.
Example 1: Find the coordinates of points \begin{align*}Q\end{align*}
Solution: In order to get to \begin{align*}Q\end{align*}
\begin{align*}Q=(3,2)\end{align*}
The coordinates of \begin{align*}R\end{align*}
\begin{align*}R=(5,2).\end{align*}
Words of Wisdom from the Graphing Plane
Not all axes will be labeled for you. There will be many times you are required to label your own axes. Some problems may require you to graph only the first quadrant. Others need two or all four quadrants. The tic marks do not always count by ones. They can be marked in increments of 2, 5, or even \begin{align*}\frac{1}{2}\end{align*}
The increments by which you count your axes should MAXIMIZE the clarity of the graph.
In Lesson 1.6, you learned the vocabulary words relation, function, domain, and range.
A relation is a set of ordered pairs.
A function is a relation in which every \begin{align*}x\end{align*}
The set of all possible \begin{align*}x\end{align*}
The set of all possible \begin{align*}y\end{align*}
Graphing Given Tables and Rules
If you kept track of the amount of money you earned for different hours of babysitting, you created a relation. You can graph the information in this table to visualize the relationship between these two variables.
\begin{align*}& \text{Hours} && 4 && 5 && 10 && 12 && 16 && 18\\ & \text{Total \$} && 12 && 15 && 30 && 36 && 48 && 54\end{align*}
The domain of the situation would be all positive real numbers. You can babysit for a fractional amount of time but not a negative amount of time. The domain would also be all positive real numbers. You can earn fractional money, but not negative money.
If you read a book and can read twenty pages an hour, there is a relationship between how many hours you read and how many pages you read. You may even know that you could write the formula as either:
\begin{align*}n & = 20 \cdot h && n = \text{number of pages;} && h = \text{time measured in hours. OR...}\\ h & = \frac{n}{20}\end{align*}
To graph this relation, you could make a chart. By picking values for the number of hours, you can determine the number of pages read. By graphing these coordinates, you can visualize the relation.
Hours  Pages 

1  20 
1.5  30 
2  40 
3.5  70 
5  100 
This relation appears to form a straight line. Therefore, the relationship between the total number of read pages and the number of hours can be called linear. The study of linear relationships is the focus of this chapter.
Practice Set
Sample explanations for some of the practice exercises below are available by viewing the following video. Note that there is not always a match between the number of the practice exercise in the video and the number of the practice exercise listed in the following exercise set. However, the practice exercise is the same in both. CK12 Basic Algebra: The Coordinate Plane (6:50)
In questions 1 – 6, identify the coordinate of the given letter.
 D
 A
 F
 E
 B
 C
Graph the following ordered pairs on one Cartesian plane. Identify the quadrant in which each ordered pair is located.
 (4, 2)
 (–3, 5.5)
 (4, –4)
 (–2, –3)

\begin{align*}\left (\frac{1}{2}, –\frac{3}{4}\right )\end{align*}
(12,–34)  (–0.75, 1)
 \begin{align*}\left (2\frac{1}{2}, 6\right )\end{align*}
 (1.60, 4.25)
In 15 – 22, using the directions given in each problem, find and graph the coordinates on a Cartesian plane.
 Six left, four down
 Onehalf right, onehalf up
 Three right, five down
 Nine left, seven up
 Four and onehalf left, three up
 Eight right, two up
 One left, one down
 One right, threequarter down
 Plot the vertices of triangle \begin{align*}ABC:(0, 0),(4, 3),(6, 2)\end{align*}.
 The following three points are three vertices of square \begin{align*}ABCD\end{align*}. Plot them on a coordinate plane then determine what the coordinates of the fourth point, \begin{align*}D\end{align*}, would be. Plot that fourth point and label it. \begin{align*}A(4,4) \ B(3,4) \ C(3,3)\end{align*}
 Does the ordered pair (2, 0) lie in a quadrant? Explain your thinking.
 Why do you think (0, 0) is called the origin?
 Becky has a large bag of M&Ms that she knows she should share with Jaeyun. Jaeyun has a packet of Starburst candy. Becky tells Jaeyun that for every Starburst he gives her, she will give him three M&Ms in return. If \begin{align*}x\end{align*} is the number of Starburst that Jaeyun gives Becky, and \begin{align*}y\end{align*} is the number of M&Ms he gets in return, then complete each of the following.
 Write an algebraic rule for \begin{align*}y\end{align*} in terms of \begin{align*}x\end{align*}.
 Make a table of values for \begin{align*}y\end{align*} with \begin{align*}x\end{align*} values of 0, 1, 2, 3, 4, 5.
 Plot the function linking \begin{align*}x\end{align*} and \begin{align*}y\end{align*} on the following scale \begin{align*}0 \le x \le 10,0 \le y \le 10\end{align*}.
 Consider the rule: \begin{align*}y=\frac{1}{4} x+8\end{align*}. Make a table. Then graph the relation.
 Ian has the following collection of data. Graph the ordered pairs and make a conclusion from the graph.
Year  % of Men Employed in the United States 

1973  75.5 
1980  72.0 
1986  71.0 
1992  69.8 
1997  71.3 
2002  69.7 
2005  69.6 
2007  69.8 
2009  64.5 
Mixed Review
 Find the sum: \begin{align*}\frac{3}{8}+\frac{1}{5}\frac{5}{9}\end{align*}.
 Solve for \begin{align*}m: 0.05m+0.025(6000m)=512\end{align*}.
 Solve the proportion for \begin{align*}u: \frac{16}{u8}=\frac{36}{u}\end{align*}.
 What does the Additive Identity Property allow you to do when solving an equation?
 Shari has 28 apples. Jordan takes \begin{align*}\frac{1}{4}\end{align*} of the apples. Shari then gives away 3 apples. How many apples does Shari have?
 The perimeter of a triangle is given by the formula \begin{align*}Perimeter=a+b+c\end{align*}, where \begin{align*}a, b,\end{align*} and \begin{align*}c\end{align*} are the lengths of the sides of a triangle. The perimeter of \begin{align*}\triangle ABC\end{align*} is 34 inches. One side of the triangle is 12 inches. A second side is 7 inches. How long is the remaining side of the triangle?
 Evaluate \begin{align*}\frac{y^216+10y+2x}{2}\end{align*}, for \begin{align*}x=2\end{align*} and \begin{align*}y=2.\end{align*}