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3.1 Linear Equations in Two Variables; The Rectangular Coordinate System

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### 3.1 Linear Equations in Two Variables; The Rectangular Coordinate System

Interpret graphs.

Recall that a bar graphis used to show comparisons. It consists of a series of bars (or simulations of bars) arranged either vertically or horizontally. In a bar graph, values from two categories are paired with each other.

A line graph is used to show changes or trends in data over time. To form a line graph,we connect a series of points representing data with line segments.

Slide 3.1-4

CLASSROOM EXAMPLE 1

Interpreting a Line Graph

Refer to the line graph below.

Estimate the average price of a gallon of gasoline in 2001.

About how much did the

average price of a gallon

of gasoline decrease

from 2001 to 2002?

Solution:

about $1.45

about $0.10

Slide 3.1-5

Some examples of linear equations in two variables in this form, called standard form, are

and

Linear equations in two variables

Other linear equations in two variables, such as

and

are not written in standard form, but could be. We discuss the forms of linear equations in more detail in Section 3.4.

Interpret graphs. (cont’d)

Linear Equation in Two Variables

A linear equation in two variables is an equation that can be written in the form

where A, B, andCare real numbers and Aand Bare not both 0.

Slide 3.1-6

y-value

x-value

Write a solution as an ordered pair.

A solution of a linear equation in two variables requires two numbers, one for each variable. For example, a true statement results when we replace x with 2 and y with 13 in the equation

since

The pair of numbers x = 2 and y = 13 gives a solution of the equation. The phrase “x = 2 and y = 13” is abbreviated

The x-value is always given first. A pair of numbers such as (2,13) is called an ordered pair.The order in which the numbers are written is important. The ordered pairs (2,13) and (13,2) are not the same. The second pair indicates that x = 13 and y = 2. For ordered pairs to be equal, their x-coordinates must be equal and their y-coordinates must be equal.

Slide 3.1-8

Decide whether a given ordered pair is a solution of a given equation.

We substitute the x-and y- values of an ordered pair into a linear equation in two variables to see whether the ordered pair is a solution. An ordered pair that is a solution of an equation is said to satisifythe equation.

Infinite numbers of ordered pairs can satisfy a linear equation in two variables.

When listing ordered pairs, be sure to always list the x-value first.

Slide 3.1-10

CLASSROOM EXAMPLE 2

Deciding Whether Ordered Pairs Are Solutions of an Equation

Decide whether each ordered pair is a solution of the equation.

Solution:

No

Yes

Slide 3.1-11

CLASSROOM EXAMPLE 3

Completing Ordered Pairs

Complete each ordered pair for the equation.

Solution:

Slide 3.1-13

Complete a table of values.

Ordered pairs are often displayed in a table of values. Although we usually write tables of values vertically, they may be written horizontally.

Slide 3.1-15

CLASSROOM EXAMPLE 4

Completing Tables of Values

Complete the table of values for the equation. Write the results as ordered pairs.

Solution:

Slide 3.1-16

Plot ordered pairs.

Every linear intwovariables equation has an infinite number of ordered pairs (x, y) as solutions. Each choice of a number for one variable leads to a particular real number for the other variable.

To graph these solutions, represented as ordered pairs (x,y), we need two number lines, one for each variable. The two number lines are drawn as shown below. The horizontal number line is called the

x-axisandthe vertical line is called

the y-axis. Together, these axis

form a rectangular coordinate

system, also called the Cartesian

coordinate system.

Slide 3.1-18

In a plane, bothnumbers in the ordered pair are needed to locate a point. The ordered pair is a name for the point.

Plot ordered pairs. (cont’d)

The coordinate system is divided into four regions, called quadrants. These quadrants are numbered counterclockwise, starting with the one in the top right quadrant.

Points on the axes themselves are not in any quadrant.

The point at which the x-axis and y-axis meet is called the origin, labeled 0 on the previous diagram. This is the point corresponding to (0, 0).

The x-axis and y-axis determine a plane — a flat surface illustrated by a sheet of paper. By referring to the two axes, we can associate every point in the plane with an ordered pair. The numbers in the ordered pair are called the coordinatesof the point.

Slide 3.1-19

Plot ordered pairs. (cont’d)

For example, locate the point associated with the ordered pair (2,3) by starting at the origin.

Since the x-coordinate is 2, go 2 units to the right along the x-axis.

Since the y-coordinate is 3, turn and go up 3 units on a line parallel to the y-axis.

The point (2,3) is plottedin the figure to the right. From now on the point with x-coordinate 2 and y-coordinate 3 will be referred to as point (2,3).

Slide 3.1-20

CLASSROOM EXAMPLE 6

Completing Ordered Pairs to Estimate the Number of Twin Births

Complete the table of ordered pairs for the equation,

where x = year and y = number of twin births in thousands. Round answers to the nearest whole number. Interpret the results for 2005.

Solution:

There were about 134 thousand twin births in the U.S. in 2005.

Slide 3.1-22

Do not assume that this scatter diagram or resulting equation would provide reliable data for other years, since the data for those years may not follow the same pattern.

Plot ordered pairs. (cont’d)

The ordered pairs of twin births in the U.S. for 2001 until 2006 are graphed to the right. This graph of ordered pairs of data is called a scatter diagram. Notice how the how the axes are labeled: x represents the year, andyrepresents the number of twin births in thousands.

A scatter diagram enables us to tell whether two quantities are related to each other. These plotted points could be connected to form a straight line, so the variables x(years) andy (number of births have a linear relationship.

Slide 3.1-23

Plot ordered pairs. (cont’d)

Think of ordered pairs as representing an input value xand an output value y. If we input xinto the equation, the output is y. We encounter many examples of this type of relationship every day.

The cost to fill a tank with gasoline depends on how many gallons are needed; the number of gallons is the input, and

the cost is the output

The distance traveled depends on the traveling time; input

a time and the output is a distance.

The growth of a plant depends on the amount of sun it gets;

the input is the amount of sun, and the output is growth.

Slide 3.1-24

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