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Statistics

12.1 Linear Equations

Statistics12.1 Linear Equations

12.1 Linear Equations

Linear regression for two variables is based on a linear equation with one independent variable. The equation has the form

y = a + b x y=a+bx


where a and b are constant numbers.

The variable x is the independent variable; y is the dependent variable. Typically, you choose a value to substitute for the independent variable and then solve for the dependent variable.

Example 12.1

The following examples are linear equations.

y=3+2xy=3+2x
y=–0.01+1.2xy=–0.01+1.2x

Try It 12.1

Is the following an example of a linear equation?

y = –0.125 – 3.5x

The graph of a linear equation of the form y = a + bx is a straight line. Any line that is not vertical can be described by this equation.

Example 12.2

Graph the equation y = –1 + 2x.

Figure 12.2

Try It 12.2

Is the following an example of a linear equation? Why or why not?

Figure 12.3

Example 12.3

Aaron’s Word Processing Service does word processing. The rate for services is $32 per hour plus a $31.50 one-time charge. The total cost to a customer depends on the number of hours it takes to complete the job.

Problem

Find the equation that expresses the total cost in terms of the number of hours required to complete the job.

Try It 12.3

Emma’s Extreme Sports hires hang-gliding instructors and pays them a fee of $50 per class, as well as $20 per student in the class. The total cost Emma pays depends on the number of students in a class. Find the equation that expresses the total cost in terms of the number of students in a class.

Slope and y-interceptof a Linear Equation

For the linear equation y = a + bx, b = slope and a = y-inttercept. From algebra, recall that the slope is a number that describes the steepness of a line; the y-intercept is the y-coordinate of the point (0, a), where the line crosses the y-axis.

Please note that in previous courses you learned y=mx+b y=mx+b was the slope-intercept form of the equation, where m represented the slope and b represented the y-intercept. In this text, the form y=a+bx y=a+bx is used, where a is the y-intercept and b is the slope. The key is remembering the coefficient of x is the slope, and the constant number is the y-intercept.

Figure 12.4 Three possible graphs of y = a + bx. (a) If b > 0, the line slopes upward to the right. (b) If b = 0, the line is horizontal. (c) If b < 0, the line slopes downward to the right.

Example 12.4

Svetlana tutors to make extra money for college. For each tutoring session, she charges a one-time fee of $25 plus $15 per hour of tutoring. A linear equation that expresses the total amount of money Svetlana earns for each session she tutors is y = 25 + 15x.

Problem

What are the independent and dependent variables? What is the y-intercept, and what is the slope? Interpret them using complete sentences.

Try It 12.4

Ethan repairs household appliances such as dishwashers and refrigerators. For each visit, he charges $25 plus $20 per hour of work. A linear equation that expresses the total amount of money Ethan earns per visit is y = 25 + 20x.

What are the independent and dependent variables? What is the y-intercept, and what is the slope? Interpret them using complete sentences.

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