Graphs

32 Use the Rectangular Coordinate System

Learning Objectives

By the end of this section, you will be able to:

  • Plot points in a rectangular coordinate system
  • Verify solutions to an equation in two variables
  • Complete a table of solutions to a linear equation
  • Find solutions to a linear equation in two variables

Before you get started, take this readiness quiz.

  1. Evaluate x+3 when x=-1.
    If you missed this problem, review (Figure).
  2. Evaluate 2x-5y when x=3 and y=-2.
    If you missed this problem, review (Figure).
  3. Solve for y: 40-4y=20.
    If you missed this problem, review (Figure).

Plot Points on a Rectangular Coordinate System

Just like maps use a grid system to identify locations, a grid system is used in algebra to show a relationship between two variables in a rectangular coordinate system. The rectangular coordinate system is also called the xy-plane or the ‘coordinate plane’.

The horizontal number line is called the x-axis. The vertical number line is called the y-axis. The x-axis and the y-axis together form the rectangular coordinate system. These axes divide a plane into four regions, called quadrants. The quadrants are identified by Roman numerals, beginning on the upper right and proceeding counterclockwise. See (Figure).

‘Quadrant’ has the root ‘quad,’ which means ‘four.’

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 7 to 7. The top-right portion of the plane is labeled "I", the top-left portion of the plane is labeled "II", the bottom-left portion of the plane is labeled "III" and the bottom-right portion of the plane is labeled "IV".

In the rectangular coordinate system, every point is represented by an ordered pair. The first number in the ordered pair is the x-coordinate of the point, and the second number is the y-coordinate of the point.

Ordered Pair

An ordered pair, \left(x,y\right), gives the coordinates of a point in a rectangular coordinate system.

The ordered pair x y is labeled with the first coordinate x labeled as "x-coordinate" and the second coordinate y labeled as "y-coordinate".

The first number is the x-coordinate.

The second number is the y-coordinate.

The phrase ‘ordered pair’ means the order is important. What is the ordered pair of the point where the axes cross? At that point both coordinates are zero, so its ordered pair is \left(0,0\right). The point \left(0,0\right) has a special name. It is called the origin.

The Origin

The point \left(0,0\right) is called the origin. It is the point where the x-axis and y-axis intersect.

We use the coordinates to locate a point on the xy-plane. Let’s plot the point \left(1,3\right) as an example. First, locate 1 on the x-axis and lightly sketch a vertical line through x=1. Then, locate 3 on the y-axis and sketch a horizontal line through y=3. Now, find the point where these two lines meet—that is the point with coordinates \left(1,3\right).

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. An arrow starts at the origin and extends right to the number 2 on the x-axis. The point (1, 3) is plotted and labeled. Two dotted lines, one parallel to the x-axis, the other parallel to the y-axis, meet perpendicularly at 1, 3. The dotted line parallel to the x-axis intercepts the y-axis at 3. The dotted line parallel to the y-axis intercepts the x-axis at 1.

Notice that the vertical line through x=1 and the horizontal line through y=3 are not part of the graph. We just used them to help us locate the point \left(1,3\right).

Plot each point in the rectangular coordinate system and identify the quadrant in which the point is located:

\left(-5,4\right)\left(-3,-4\right)\left(2,-3\right)\left(-2,3\right)\left(3,\frac{5}{2}\right).

Solution

The first number of the coordinate pair is the x-coordinate, and the second number is the y-coordinate.

  1. Since x=-5, the point is to the left of the y-axis. Also, since y=4, the point is above the x-axis. The point \left(-5,4\right) is in Quadrant II.
  2. Since x=-3, the point is to the left of the y-axis. Also, since y=-4, the point is below the x-axis. The point \left(-3,-4\right) is in Quadrant III.
  3. Since x=2, the point is to the right of the y-axis. Since y=-3, the point is below the x-axis. The point \left(2,-3\right) is in Quadrant lV.
  4. Since x=-2, the point is to the left of the y-axis. Since y=3, the point is above the x-axis. The point \left(-2,3\right) is in Quadrant II.
  5. Since x=3, the point is to the right of the y-axis. Since y=\frac{5}{2}, the point is above the x-axis. (It may be helpful to write \frac{5}{2} as a mixed number or decimal.) The point \left(3,\frac{5}{2}\right) is in Quadrant I.
    The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 7 to 7. The points (negative 5, 4), (negative 2, 3), (negative 3, negative 4), (3, five halves), and (2, negative 3) are plotted and labeled.

Plot each point in a rectangular coordinate system and identify the quadrant in which the point is located:

\left(-2,1\right)\left(-3,-1\right)\left(4,-4\right)\left(-4,4\right)\left(-4,\frac{3}{2}\right).

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The point (negative 2, 1) is plotted and labeled "a". The point (negative 3, negative 1) is plotted and labeled "b". The point (4, negative 4) is plotted and labeled "c". The point (negative 4, negative one half) is plotted and labeled “d”.

Plot each point in a rectangular coordinate system and identify the quadrant in which the point is located:

\left(-4,1\right)\left(-2,3\right)\left(2,-5\right)\left(-2,5\right)\left(-3,\frac{5}{2}\right).

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The point (negative 4, 1) is plotted and labeled "a". The point (negative 2, 3) is plotted and labeled "b". The point (2, negative 5) is plotted and labeled "c". The point (negative 3, 2 and one half) is plotted and labeled “d”.

How do the signs affect the location of the points? You may have noticed some patterns as you graphed the points in the previous example.

For the point in (Figure) in Quadrant IV, what do you notice about the signs of the coordinates? What about the signs of the coordinates of points in the third quadrant? The second quadrant? The first quadrant?

Can you tell just by looking at the coordinates in which quadrant the point \left(-2,5\right) is located? In which quadrant is \left(2,-5\right) located?

Quadrants

We can summarize sign patterns of the quadrants in this way.

\begin{array}{cccccccccc}\hfill \text{Quadrant I}\hfill & & & \hfill \text{Quadrant II}\hfill & & & \hfill \text{Quadrant III}\hfill & & & \hfill \text{Quadrant IV}\hfill \\ \hfill \left(x,y\right)\hfill & & & \hfill \left(x,y\right)\hfill & & & \hfill \left(x,y\right)\hfill & & & \hfill \left(x,y\right)\hfill \\ \hfill \left(+,+\right)\hfill & & & \hfill \left(\text{−},+\right)\hfill & & & \hfill \left(\text{−},\text{−}\right)\hfill & & & \hfill \left(+,\text{−}\right)\hfill \end{array}

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 7 to 7. The graph shows the x y-coordinate plane. The x and y-axis each run from -7 to 7. The top-right portion of the plane is labeled "I" and "ordered pair +, +", the top-left portion of the plane is labeled "II" and "ordered pair -, +", the bottom-left portion of the plane is labelled "III" "ordered pair -, -" and the bottom-right portion of the plane is labeled "IV" and "ordered pair +, -".

What if one coordinate is zero as shown in (Figure)? Where is the point \left(0,4\right) located? Where is the point \left(-2,0\right) located?

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. Points (0, 4) and (negative 2, 0) are plotted and labeled.

The point \left(0,4\right) is on the y-axis and the point \left(-2,0\right) is on the x-axis.

Points on the Axes

Points with a y-coordinate equal to 0 are on the x-axis, and have coordinates \left(a,0\right).

Points with an x-coordinate equal to 0 are on the y-axis, and have coordinates \left(0,b\right).

Plot each point:

\left(0,5\right)\left(4,0\right)\left(-3,0\right)\left(0,0\right)\left(0,-1\right).

Solution
  1. Since x=0, the point whose coordinates are \left(0,5\right) is on the y-axis.
  2. Since y=0, the point whose coordinates are \left(4,0\right) is on the x-axis.
  3. Since y=0, the point whose coordinates are \left(-3,0\right) is on the x-axis.
  4. Since x=0 and y=0, the point whose coordinates are \left(0,0\right) is the origin.
  5. Since x=0, the point whose coordinates are \left(0,-1\right) is on the y-axis.
    The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 7 to 7. The points (negative 3, 0), (0, 0), (0, negative 1), (0, 5), and (4, 0) are plotted and labeled.

Plot each point:

\left(4,0\right)\left(-2,0\right)\left(0,0\right)\left(0,2\right)\left(0,-3\right).

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The points (4, 0), (negative 2, 0), (0, 0), (0, 2), and (0, negative 3) are plotted and labeled.

Plot each point:

\left(-5,0\right)\left(3,0\right)\left(0,0\right)\left(0,-1\right)\left(0,4\right).

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The points (negative 5, 0), (3, 0), (0, 0), (0, negative 1), and (0, 4) are plotted and labeled.

In algebra, being able to identify the coordinates of a point shown on a graph is just as important as being able to plot points. To identify the x-coordinate of a point on a graph, read the number on the x-axis directly above or below the point. To identify the y-coordinate of a point, read the number on the y-axis directly to the left or right of the point. Remember, when you write the ordered pair use the correct order, \left(x,y\right).

Name the ordered pair of each point shown in the rectangular coordinate system.

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The points (4, 0), (negative 2, 0), (0, 0), (0, 2), and (0, negative 3) are plotted and labeled A, B, C, D, and E, respectively.

Solution

Point A is above -3 on the x-axis, so the x-coordinate of the point is -3.

  • The point is to the left of 3 on the y-axis, so the y-coordinate of the point is 3.
  • The coordinates of the point are \left(-3,3\right).

Point B is below -1 on the x-axis, so the x-coordinate of the point is -1.

  • The point is to the left of -3 on the y-axis, so the y-coordinate of the point is -3.
  • The coordinates of the point are \left(-1,-3\right).

Point C is above 2 on the x-axis, so the x-coordinate of the point is 2.

  • The point is to the right of 4 on the y-axis, so the y-coordinate of the point is 4.
  • The coordinates of the point are \left(2,4\right).

Point D is below 4 on the x-axis, so the x-coordinate of the point is 4.

  • The point is to the right of -4 on the y-axis, so the y-coordinate of the point is -4.
  • The coordinates of the point are \left(4,-4\right).

Point E is on the y-axis at y=-2. The coordinates of point E are \left(0,-2\right).

Point F is on the x-axis at x=3. The coordinates of point F are \left(3,0\right).

Name the ordered pair of each point shown in the rectangular coordinate system.

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The points (4, 0), (negative 2, 0), (0, 0), (0, 2), and (0, negative 3) are plotted and labeled A, B, C, D, and E, respectively.

A: \left(5,1\right) B: \left(-2,4\right) C: \left(-5,-1\right) D: \left(3,-2\right) E: \left(0,-5\right) F: \left(4,0\right)

Name the ordered pair of each point shown in the rectangular coordinate system.

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The points (negative 5, 0), (3, 0), (0, 0), (0, negative 1), and (0, 4) are plotted and labeled A, B, C, D, and E, respectively.

A: \left(4,2\right) B: \left(-2,3\right) C: \left(-4,-4\right) D: \left(3,-5\right) E: \left(-3,0\right) F: \left(0,2\right)

Verify Solutions to an Equation in Two Variables

Up to now, all the equations you have solved were equations with just one variable. In almost every case, when you solved the equation you got exactly one solution. The process of solving an equation ended with a statement like x=4. (Then, you checked the solution by substituting back into the equation.)

Here’s an example of an equation in one variable, and its one solution.

\begin{array}{ccc}\hfill 3x+5& =\hfill & 17\hfill \\ \hfill 3x& =\hfill & 12\hfill \\ \hfill x& =\hfill & 4\hfill \end{array}

But equations can have more than one variable. Equations with two variables may be of the form Ax+By=C. Equations of this form are called linear equations in two variables.

Linear Equation

An equation of the form Ax+By=C, where A and B are not both zero, is called a linear equation in two variables.

Notice the word line in linear. Here is an example of a linear equation in two variables, x and y.

In this figure, we see the linear equation Ax plus By equals C. Below this is the equation x plus 4y equals 8. Below this are the values A equals 1, B equals 4, and C equals 8.

The equation y=-3x+5 is also a linear equation. But it does not appear to be in the form Ax+By=C. We can use the Addition Property of Equality and rewrite it in Ax+By=C form.

\phantom{\rule{2.0em}{0ex}}y\phantom{\rule{0.5em}{0ex}}=\phantom{\rule{0.5em}{0ex}}-3x+5
Add to both sides. y+3x\phantom{\rule{0.5em}{0ex}}=\phantom{\rule{0.5em}{0ex}}-3x+5+3x
Simplify. y+3x\phantom{\rule{0.5em}{0ex}}=\phantom{\rule{0.5em}{0ex}}5
Use the Commutative Property to put it in Ax+By=C form. 3x+y\phantom{\rule{0.5em}{0ex}}=\phantom{\rule{0.5em}{0ex}}5

By rewriting y=-3x+5 as 3x+y=5, we can easily see that it is a linear equation in two variables because it is of the form Ax+By=C. When an equation is in the form Ax+By=C, we say it is in standard form.

Standard Form of Linear Equation

A linear equation is in standard form when it is written Ax+By=C.

Most people prefer to have A, B, and C be integers and A\ge 0 when writing a linear equation in standard form, although it is not strictly necessary.

Linear equations have infinitely many solutions. For every number that is substituted for x there is a corresponding y value. This pair of values is a solution to the linear equation and is represented by the ordered pair \left(x,y\right). When we substitute these values of x and y into the equation, the result is a true statement, because the value on the left side is equal to the value on the right side.

Solution of a Linear Equation in Two Variables

An ordered pair \left(x,y\right) is a solution of the linear equation Ax+By=C, if the equation is a true statement when the x– and y-values of the ordered pair are substituted into the equation.

Determine which ordered pairs are solutions to the equation x+4y=8.

\left(0,2\right)\left(2,-4\right)\left(-4,3\right)

Solution

Substitute the x– and y-values from each ordered pair into the equation and determine if the result is a true statement.

This figure has three columns. At the top of the first column is the ordered pair (0, 2). Below this are the values x equals 0 and y equals 2. Below this is the equation x plus 4y equals 8. Below this is the same equation with 0 and 2 substituted for x and y: 0 plus 4 times 2 might equal 8. Below this is 0 plus 8 might equal 8. Below this is 8 equals 8 with a check mark next to it. Below this is the sentence “(0, 2) is a solution.” At the top of the second column is the ordered pair (2, negative 4). Below this are the values x equals 2 and y equals negative 4. Below this is the equation x plus 4y equals 8. Below this is the same equation with 2 and negative 4 substituted for x and y: 2 plus 4 times negative 4 might equal 8. Below this is 2 plus negative 16 might equal 8. Below this is negative 14 does not equal 8. Below this is the sentence: “(2, negative 4) is not a solution.” At the top of the third column is the ordered pair (negative 4, 3). Below this are the values x equals negative 4 and y equals 3. Below this is the equation x plus 4y equals 8. Below this is the same equation with negative 4 and 3 substituted for x and y: negative 4 plus 4 times 3 might equal 8. Below this is negative 4 plus 12 might equal 8. Below this is 8 equals 8 with a check mark next to it. Below this is the sentence: “(negative 4, 3) is a solution.”

Which of the following ordered pairs are solutions to 2x+3y=6?

\left(3,0\right)\left(2,0\right)\left(6,-2\right)

a, c

Which of the following ordered pairs are solutions to the equation 4x-y=8?

\left(0,8\right)\left(2,0\right)\left(1,-4\right)

b, c

Which of the following ordered pairs are solutions to the equation y=5x-1?

\left(0,-1\right)\left(1,4\right)\left(-2,-7\right)

Solution

Substitute the x– and y-values from each ordered pair into the equation and determine if it results in a true statement.

This figure has three columns. At the top of the first column is the ordered pair (0, negative 1). Below this are the values x equals 0 and y equals negative 1. Below this is the equation y equals 5x minus 1. Below this is the same equation with 0 and negative 1 substituted for x and y: negative 1 might equal 5 times 0 minus 1. Below this is negative 1 might equal 0 minus 1. Below this is negative 1 equals negative 1 with a check mark next to it. Below this is the sentence: “(0, negative 1) is a solution.” At the top of the second column is the ordered pair (1, 4). Below this are the values x equals 1 and y equals 4. Below this is the equation y equals 5x minus 1. Below this is the same equation with 1 and 4 substituted for x and y: 4 might equal 5 times 1 minus 1. Below this is 4 might equal 5 minus 1. Below this is 4 equals 4 with a check mark next to it. Below this is the sentence: “(1, 4) is a solution.” At the top of the right column is the ordered pair (negative 2, negative 7). Below this are the values x equals negative 2 and y equals negative 7. Below this is the equation y equals 5x minus 1. Below this is the same equation with negative 2 and negative 7 substituted for x and y: negative 7 might equal 5 times negative 2 minus 1. Below this is negative 7 might equal negative 10 minus 1. Below this is negative 7 does not equal negative 11. Below this is the sentence: “(negative 2, negative 7) is not a solution.”

Which of the following ordered pairs are solutions to the equation y=4x-3?

\left(0,3\right)\left(1,1\right)\left(-1,-1\right)

b

Which of the following ordered pairs are solutions to the equation y=-2x+6?

\left(0,6\right)\left(1,4\right)\left(-2,-2\right)

a, b

Complete a Table of Solutions to a Linear Equation in Two Variables

In the examples above, we substituted the x– and y-values of a given ordered pair to determine whether or not it was a solution to a linear equation. But how do you find the ordered pairs if they are not given? It’s easier than you might think—you can just pick a value for x and then solve the equation for y. Or, pick a value for y and then solve for x.

We’ll start by looking at the solutions to the equation y=5x-1 that we found in (Figure). We can summarize this information in a table of solutions, as shown in (Figure).

y=5x-1
x y \left(x,y\right)
0 -1 \left(0,-1\right)
1 4 \left(1,4\right)

To find a third solution, we’ll let x=2 and solve for y.

The figure shows the steps to solve for y when x equals 2 in the equation y equals 5 x minus 1. The equation y equals 5 x minus 1 is shown. Below it is the equation with 2 substituted in for x which is y equals 5 times 2 minus 1. To solve for y first multiply so that the equation becomes y equals 10 minus 1 then subtract so that the equation is y equals 9.

The ordered pair \left(2,9\right) is a solution to y=5x-1. We will add it to (Figure).

y=5x-1
x y \left(x,y\right)
0 -1 \left(0,-1\right)
1 4 \left(1,4\right)
2 9 \left(2,9\right)

We can find more solutions to the equation by substituting in any value of x or any value of y and solving the resulting equation to get another ordered pair that is a solution. There are infinitely many solutions of this equation.

Complete (Figure) to find three solutions to the equation y=4x-2.

y=4x-2
x y \left(x,y\right)
0
-1
2
Solution

Substitute x=0, x=-1, and x=2 into y=4x-2.

This figure has three columns. At the top of the first column is the value x equals 0. Below this is the equation y equals 4x minus 2. Below this is the same equation with 0 substituted for x: y equals 4 times 0 minus 2. Below this is y equals 0 minus 2. Below this is y equals negative 2. Below this is the ordered pair (0, negative 2). At the top of the second column is the value x equals negative 1. Below this is the equation y equals 4x minus 2. Below this is the same equation with negative 1 substituted for x: y equals 4 times minus 1 minus 2. Below this is y equals negative 4 minus 2. Below this is y equals negative 6. Below this is the ordered pair (negative 1, negative 6). At the top of the third column is the value x equals 2. Below this is the equation y equals 4x minus 2. Below this is the same equation with 2 substituted for x: y equals 4 times 2 minus 2. Below this is y equals 8 minus 2. Below this is y equals 6. Below this is the ordered pair (2, 6).

The results are summarized in (Figure).

y=4x-2
x y \left(x,y\right)
0 -2 \left(0,-2\right)
-1 -6 \left(-1,-6\right)
2 6 \left(2,6\right)

Complete the table to find three solutions to this equation: y=3x-1.

y=3x-1
x y \left(x,y\right)
0
-1
2
y=3x-1
x y \left(x,y\right)
0 -1 \left(0,-1\right)
-1 -4 \left(-1,-4\right)
2 5 \left(2,5\right)

Complete the table to find three solutions to this equation: y=6x+1.

y=6x+1
x y \left(x,y\right)
0
1
-2
y=6x+1
x y \left(x,y\right)
0 1 \left(0,1\right)
1 7 \left(1,7\right)
-2 -11 \left(-2,-11\right)

Complete (Figure) to find three solutions to the equation 5x-4y=20.

5x-4y=20
x y \left(x,y\right)
0
0
5
Solution

Substitute the given value into the equation 5x-4y=20 and solve for the other variable. Then, fill in the values in the table.

This figure has three columns. At the top of the first column is the value x equals 0. Below this is the equation 5x minus 4y equals 20. Below this is the same equation with 0 substituted for x: 5 times 0 minus 4y equals 20. Below this is 0 minus 4y equals 20. Below this is negative 4y equals 20. Below this is y equals negative 5. Below this is the ordered pair (0, negative 5). At the top of the second column is the value y equals 0. Below this is the equation 5x minus 4y equals 20. Below this is the same equation with 0 substituted for y: 5x minus 4 times 0 equals 20. Below this is 5x minus 0 equals 20. Below this is 5x equals 20. Below this is x equals 4. Below this is the ordered pair (4, 0). At the top of the third column is the value y equals 5. Below this is the equation 5x minus 47 equals 20. Below this is the same equation with 5 substituted for y: 5x minus 4 times 5 equals 20. Below this is the equation 5x minus 20 equals 20. Below this is 5x equals 40. Below this is x equals 8. Below this is the ordered pair (8, 5).

The results are summarized in (Figure).

5x-4y=20
x y \left(x,y\right)
0 -5 \left(0,-5\right)
4 0 \left(4,0\right)
8 5 \left(8,5\right)

Complete the table to find three solutions to this equation: 2x-5y=20.

2x-5y=20
x y \left(x,y\right)
0
0
-5
2x-5y=20
x y \left(x,y\right)
0 -4 \left(0,-4\right)
10 0 \left(10,0\right)
-5 -6 \left(-5,-6\right)

Complete the table to find three solutions to this equation: 3x-4y=12.

3x-4y=12
x y \left(x,y\right)
0
0
-4
3x-4y=12
x y \left(x,y\right)
0 -3 \left(0,-3\right)
4 0 \left(4,0\right)
-4 -6 \left(-4,-6\right)

Find Solutions to a Linear Equation

To find a solution to a linear equation, you really can pick any number you want to substitute into the equation for x or y. But since you’ll need to use that number to solve for the other variable it’s a good idea to choose a number that’s easy to work with.

When the equation is in y-form, with the y by itself on one side of the equation, it is usually easier to choose values of x and then solve for y.

Find three solutions to the equation y=-3x+2.

Solution

We can substitute any value we want for x or any value for y. Since the equation is in y-form, it will be easier to substitute in values of x. Let’s pick x=0, x=1, and x=-1.

. . .
. . .
Substitute the value into the equation. . . .
Simplify. . . .
Simplify. . . .
Write the ordered pair. (0, 2) (1, −1) (−1, 5)
Check.
\phantom{\rule{0.03em}{0ex}}y=-3x+2 \phantom{\rule{0.7em}{0ex}}y=-3x+2 \phantom{\rule{0.04em}{0ex}}y=-3x+2
2\stackrel{?}{=}-3\cdot 0+2 -1\stackrel{?}{=}-3\cdot 1+2 5\stackrel{?}{=}-3\left(-1\right)+2
2\stackrel{?}{=}0+2 -1\stackrel{?}{=}-3+2 5\stackrel{?}{=}3+2
2=2✓ -1=-1✓ 5=5✓

So, \left(0,2\right), \left(1,-1\right) and \left(-1,5\right) are all solutions to y=-3x+2. We show them in (Figure).

y=-3x+2
x y \left(x,y\right)
0 2 \left(0,2\right)
1 -1 \left(1,-1\right)
-1 5 \left(-1,5\right)

Find three solutions to this equation: y=-2x+3.

Answers will vary.

Find three solutions to this equation: y=-4x+1.

Answers will vary.

We have seen how using zero as one value of x makes finding the value of y easy. When an equation is in standard form, with both the x and y on the same side of the equation, it is usually easier to first find one solution when x=0 find a second solution when y=0, and then find a third solution.

Find three solutions to the equation 3x+2y=6.

Solution

We can substitute any value we want for x or any value for y. Since the equation is in standard form, let’s pick first x=0, then y=0, and then find a third point.

. . .
. . .
Substitute the value into the equation. . . .
Simplify. . . .
Solve. . . .
. . .
Write the ordered pair. (0, 3) (2, 0) \left(1,\frac{3}{2}\right)
Check.
3x+2y=6\phantom{\rule{1.3em}{0ex}} 3x+2y=6\phantom{\rule{1.3em}{0ex}} 3x+2y=6\phantom{\rule{1.3em}{0ex}}
3\cdot 0+2\cdot 3\stackrel{?}{=}6\phantom{\rule{1.3em}{0ex}} 3\cdot 2+2\cdot 0\stackrel{?}{=}6\phantom{\rule{1.3em}{0ex}} 3\cdot 1+2\cdot \frac{3}{2}\stackrel{?}{=}6\phantom{\rule{1.3em}{0ex}}
0+6\stackrel{?}{=}6\phantom{\rule{1.3em}{0ex}} 6+0\stackrel{?}{=}6\phantom{\rule{1.3em}{0ex}} 3+3\stackrel{?}{=}6\phantom{\rule{1.3em}{0ex}}
6=6✓ 6=6✓ 6=6✓

So \left(0,3\right), \left(2,0\right), and \left(1,\frac{3}{2}\right) are all solutions to the equation 3x+2y=6. We can list these three solutions in (Figure).

3x+2y=6
x y \left(x,y\right)
0 3 \left(0,3\right)
2 0 \left(2,0\right)
1 \frac{3}{2} \left(1,\frac{3}{2}\right)

Find three solutions to the equation 2x+3y=6.

Answers will vary.

Find three solutions to the equation 4x+2y=8.

Answers will vary.

Key Concepts

  • Sign Patterns of the Quadrants
    \begin{array}{cccccccccc}\text{Quadrant I}\hfill & & & \text{Quadrant II}\hfill & & & \text{Quadrant III}\hfill & & & \text{Quadrant IV}\hfill \\ \left(x,y\right)\hfill & & & \left(x,y\right)\hfill & & & \left(x,y\right)\hfill & & & \left(x,y\right)\hfill \\ \left(+,+\right)\hfill & & & \left(\text{−},+\right)\hfill & & & \left(\text{−},\text{−}\right)\hfill & & & \left(+,\text{−}\right)\hfill \end{array}
  • Points on the Axes
    • On the x-axis, y=0. Points with a y-coordinate equal to 0 are on the x-axis, and have coordinates \left(a,0\right).
    • On the y-axis, x=0. Points with an x-coordinate equal to 0 are on the y-axis, and have coordinates \left(0,b\right).
  • Solution of a Linear Equation
    • An ordered pair \left(x,y\right) is a solution of the linear equation Ax+By=C, if the equation is a true statement when the x– and y– values of the ordered pair are substituted into the equation.

Practice Makes Perfect

Plot Points in a Rectangular Coordinate System

In the following exercises, plot each point in a rectangular coordinate system and identify the quadrant in which the point is located.


\left(-4,2\right)
\left(-1,-2\right)
\left(3,-5\right)
\left(-3,5\right)
\left(\frac{5}{3},2\right)

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The point (negative 4, 2) is plotted and labeled "a". The point (negative 1, negative 2) is plotted and labeled "b". The point (3, negative 5) is plotted and labeled "c". The point (negative 3, 5) is plotted and labeled “d”. The point (5 thirds, 2) is plotted and labeled “e”.


\left(-2,-3\right)
\left(3,-3\right)
\left(-4,1\right)
\left(4,-1\right)
\left(\frac{3}{2},1\right)


\left(3,-1\right)
\left(-3,1\right)
\left(-2,2\right)
\left(-4,-3\right)
\left(1,\frac{14}{5}\right)

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The point (3, negative 1) is plotted and labeled "a". The point (negative 3, 1) is plotted and labeled "b". The point (negative 2, 2) is plotted and labeled "c". The point (negative 4, negative 3) is plotted and labeled “d”. The point (1, 14 fifths) is plotted and labeled “e”.


\left(-1,1\right)
\left(-2,-1\right)
\left(2,1\right)
\left(1,-4\right)
\left(3,\frac{7}{2}\right)

In the following exercises, plot each point in a rectangular coordinate system.


\left(-2,0\right)
\left(-3,0\right)
\left(0,0\right)
\left(0,4\right)
\left(0,2\right)

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The point (negative 2, 0) is plotted and labeled "a". The point (negative 3, 0) is plotted and labeled "b". The point (0, 0) is plotted and labeled "c". The point (0, 4) is plotted and labeled “d”. The point (0, 3) is plotted and labeled “e”.


\left(0,1\right)
\left(0,-4\right)
\left(-1,0\right)
\left(0,0\right)
\left(5,0\right)


\left(0,0\right)
\left(0,-3\right)
\left(-4,0\right)
\left(1,0\right)
\left(0,-2\right)

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The point (0, 0) is plotted and labeled "a". The point (0, negative 3) is plotted and labeled "b". The point (negative 4, 0) is plotted and labeled "c". The point (1, 0) is plotted and labeled “d”. The point (0, negative 2) is plotted and labeled “e”.


\left(-3,0\right)
\left(0,5\right)
\left(0,-2\right)
\left(2,0\right)
\left(0,0\right)

In the following exercises, name the ordered pair of each point shown in the rectangular coordinate system.

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The point (negative 4, 1) is plotted and labeled “A”. The point (negative 3, negative 4) is plotted and labeled “B”. The point (1, negative 3) is plotted and labeled “C”. The point (4, 3) is plotted and labeled “D”.

A: \left(-4,1\right) B: \left(-3,-4\right) C: \left(1,-3\right) D: \left(4,3\right)

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 10 to 10. The point (negative 4, 2) is plotted and labeled “A”. The point (3, 5) is plotted and labeled “B”. The point (negative 3, negative 2) is plotted and labeled “C”. The point (5, negative 1) is plotted and labeled “D”.
The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The point (0, negative 2) is plotted and labeled “A”. The point (negative 2, 0) is plotted and labeled “B”. The point (0, 5) is plotted and labeled “C”. The point (5, 0) is plotted and labeled “D”.

A: \left(0,-2\right) B: \left(-2,0\right) C: \left(0,5\right) D: \left(5,0\right)

The graph shows the x y-coordinate plane. The x- and y-axes each run from negative 6 to 6. The point (0, negative 1) is plotted and labeled “A”. The point (negative 1, 0) is plotted and labeled “B”. The point (4, 0) is plotted and labeled “C”. The point (0, 4) is plotted and labeled “D”.

Verify Solutions to an Equation in Two Variables

In the following exercises, which ordered pairs are solutions to the given equations?

2x+y=6

\left(1,4\right)
\left(3,0\right)
\left(2,3\right)

a, b

x+3y=9

\left(0,3\right)
\left(6,1\right)
\left(-3,-3\right)

4x-2y=8

\left(3,2\right)
\left(1,4\right)
\left(0,-4\right)

a, c

3x-2y=12

\left(4,0\right)
\left(2,-3\right)
\left(1,6\right)

y=4x+3

\left(4,3\right)
\left(-1,-1\right)
\left(\frac{1}{2},5\right)

b, c

y=2x-5

\left(0,-5\right)
\left(2,1\right)
\left(\frac{1}{2},-4\right)

y=\frac{1}{2}x-1

\left(2,0\right)
\left(-6,-4\right)
\left(-4,-1\right)

a, b

y=\frac{1}{3}x+1

\left(-3,0\right)
\left(9,4\right)
\left(-6,-1\right)

Complete a Table of Solutions to a Linear Equation

In the following exercises, complete the table to find solutions to each linear equation.

y=2x-4

x y \left(x,y\right)
0
2
-1
x y \left(x,y\right)
0 -4 \left(0,-4\right)
2 0 \left(2,0\right)
-1 -6 \left(-1,-6\right)

y=3x-1

x y \left(x,y\right)
0
2
-1

y=\text{−}x+5

x y \left(x,y\right)
0
3
-2
x y \left(x,y\right)
0 5 \left(0,5\right)
3 2 \left(3,2\right)
-2 7 \left(-2,7\right)

y=\text{−}x+2

x y \left(x,y\right)
0
3
-2

y=\frac{1}{3}x+1

x y \left(x,y\right)
0
3
6
x y \left(x,y\right)
0 1 \left(0,1\right)
3 2 \left(3,2\right)
6 3 \left(6,3\right)

y=\frac{1}{2}x+4

x y \left(x,y\right)
0
2
4

y=-\frac{3}{2}x-2

x y \left(x,y\right)
0
2
-2
x y \left(x,y\right)
0 -2 \left(0,-2\right)
2 -5 \left(2,-5\right)
-2 1 \left(-2,1\right)

y=-\frac{2}{3}x-1

x y \left(x,y\right)
0
3
-3

x+3y=6

x y \left(x,y\right)
0
3
0
x y \left(x,y\right)
0 2 \left(0,2\right)
3 4 \left(3,1\right)
6 0 \left(6,0\right)

x+2y=8

x y \left(x,y\right)
0
4
0

2x-5y=10

x y \left(x,y\right)
0
10
0
x y \left(x,y\right)
0 -2 \left(0,-2\right)
10 2 \left(10,2\right)
5 0 \left(5,0\right)

3x-4y=12

x y \left(x,y\right)
0
8
0

Find Solutions to a Linear Equation

In the following exercises, find three solutions to each linear equation.

y=5x-8

Answers will vary.

y=3x-9

y=-4x+5

Answers will vary.

y=-2x+7

x+y=8

Answers will vary.

x+y=6

x+y=-2

Answers will vary.

x+y=-1

3x+y=5

Answers will vary.

2x+y=3

4x-y=8

Answers will vary.

5x-y=10

2x+4y=8

Answers will vary.

3x+2y=6

5x-2y=10

Answers will vary.

4x-3y=12

Everyday Math

Weight of a baby. Mackenzie recorded her baby’s weight every two months. The baby’s age, in months, and weight, in pounds, are listed in the table below, and shown as an ordered pair in the third column.

Plot the points on a coordinate plane.

.

Why is only Quadrant I needed?

Age x Weight y \left(x,y\right)
0 7 (0, 7)
2 11 (2, 11)
4 15 (4, 15)
6 16 (6, 16)
8 19 (8, 19)
10 20 (10, 20)
12 21 (12, 21)


The graph shows the x y-coordinate plane. The x- and y-axes each run from 0 to 25. The points (0, 7), (2, 11), (4, 15), (6, 16), (8, 19), (10, 20) and (12, 21) are plotted and labeled.


Age and weight are only positive.

Weight of a child. Latresha recorded her son’s height and weight every year. His height, in inches, and weight, in pounds, are listed in the table below, and shown as an ordered pair in the third column.

Plot the points on a coordinate plane.

.

Why is only Quadrant I needed?

Height x Weight y \left(x,y\right)
28 22 (28, 22)
31 27 (31, 27)
33 33 (33, 33)
37 35 (37, 35)
40 41 (40, 41)
42 45 (42, 45)

Writing Exercises

Explain in words how you plot the point \left(4,-2\right) in a rectangular coordinate system.

Answers will vary.

How do you determine if an ordered pair is a solution to a given equation?

Is the point \left(-3,0\right) on the x-axis or y-axis? How do you know?

Answers will vary.

Is the point \left(0,8\right) on the x-axis or y-axis? How do you know?

Self Check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

This is a table that has six rows and four columns. In the first row, which is a header row, the cells read from left to right: “I can…,” “confidently,” “with some help,” and “no-I don’t get it!” The first column below “I can…” reads “plot points in a rectangular coordinate system,”, “identify points on a graph,” “verify solutions to an equation in two variables,” “complete a table of solutions to a linear equation,” and “find solutions to a linear equation.” The rest of the cells are blank.

If most of your checks were:

…confidently. Congratulations! You have achieved the objectives in this section. Reflect on the study skills you used so that you can continue to use them. What did you do to become confident of your ability to do these things? Be specific.

…with some help. This must be addressed quickly because topics you do not master become potholes in your road to success. In math every topic builds upon previous work. It is important to make sure you have a strong foundation before you move on. Who can you ask for help? Your fellow classmates and instructor are good resources. Is there a place on campus where math tutors are available? Can your study skills be improved?

…no, I don’t get it. This is a warning sign and you must not ignore it. You should get help right away or you will quickly be overwhelmed. See your instructor as soon as you can to discuss your situation. Together you can come up with a plan to get you the help you need.

Glossary

linear equation
A linear equation is of the form Ax+By=C, where A and B are not both zero, is called a linear equation in two variables.
ordered pair
An ordered pair \left(x,y\right) gives the coordinates of a point in a rectangular coordinate system.
origin
The point \left(0,0\right) is called the origin. It is the point where the x-axis and y-axis intersect.
quadrant
The x-axis and the y-axis divide a plane into four regions, called quadrants.
rectangular coordinate system
A grid system is used in algebra to show a relationship between two variables; also called the xy-plane or the ‘coordinate plane’.
x-coordinate
The first number in an ordered pair \left(x,y\right).
y-coordinate
The second number in an ordered pair \left(x,y\right).

License

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Elementary Algebra by OSCRiceUniversity is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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