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7.2: Introducing the Absolute Value Function

Difficulty Level: At Grade Created by: CK-12

This activity is intended to supplement Algebra I, Chapter 6, Lesson 5.

In this activity, you will examine data by comparing individual data points to the mean by finding the difference (positive or negative) and the distance from the mean, plot the distances versus the differences to examine the shape of the plot, investigate the absolute value function in the Y= register to model the relationship between the distances and the differences, and extend the investigation of absolute value equations by examining tables and graphs.

Problem 1 - Analyze the Data

The high temperatures in the first twelve days of February were: 43,49,47,42,54,55,58, 58,61,62,49,46.

Press STAT ENTER. Enter these 12 data points into L1.

Press 2nd [MODE] to return to the home screen. Press 2nd [STAT] to the ‘Math on Lists’ menu. Press 3 to select 3:mean(.

This will paste the command onto the home screen. Press 2nd [1] ) to complete the command to find the mean of L1. Press ENTER to execute.

Now that you know the mean of the temperatures, press STAT ENTER to return to the ‘statistics editor.’ Arrow to the top of L2 as shown.

Press 2nd [1]52. This will command the calculator to subtract the mean of 52 from each of the temperatures in L1.

Press ENTER to execute. What do you notice about the numbers in L2? What is the highest difference? What is the smallest difference? When are the differences negative? Positive?

Move over to L3. Examine each entry in L1 and determine is DISTANCE from the mean (how far away). Enter the distances in L3. What is the relationship between the distances and the differences from L2? Why is this so?

Set up a scatter plot to compare the distances to the differences (L3 to L2). Press 2nd Y=. Press 1 to select 1:Plot 1.

Press ENTER to turn the plot On. Arrow down to the Xlist.

Press 2nd [2] to use L2 (the differences) as the x list. Arrow down to the Ylist. Press 2nd [3] to use L3 (the distances) as the y list.

Press WINDOW. Set the window as shown.

Press GRAPH. Press TRACE to examine the relationships between the x and ycoordinates of each point. When x is positive, what happens to y?

When x is negative, what happens to y? When will y be negative? Why? When is x negative?

(sample response: x is negative whenever the temperature was lower than the mean; y will not be negative because distances are positive)

Problem 2 - Compare Data Against Equations

Press Y=. Enter the equation y=x into Y1.

Press GRAPH. What is the relationship between y=x and the scatter plot?

Return to Y=. Enter the equation y=x into Y2.

Press GRAPH. What is the relationship between y=x and the scatter plot?

Press 2nd [GRAPH] to examine the tables for Y1 and Y2. How are the values for X and Y1 related? How are the values for X and Y2 related? How are the values for Y1 and Y2 related? Where is each Y equal to zero?

Return to Y=. Arrow down to Y3. Press MATH to find the absolute value command 1:abs(. Press ENTER. This will paste the command into Y3.

Complete the function as shown. Arrow left of Y3. Press ENTER to change the graph to a ‘thick line.’

Press GRAPH. What is the relationship between y=abs(x) and the scatter plot? NOTE: In your textbook this function will be written as y=|x|.

Press 2nd [GRAPH] to examine the tables. How are the values for Y3 related to Y1 and Y2? Where is Y equal to zero?

Extension

Examine another absolute value equation. First, clear all earlier functions in Y= and enter y=x+7 into Y1. To clear functions, put cursor over the equals sign and press [CLEAR].

Press 2nd [GRAPH] Examine the table. When are the Y1 values positive? When are they negative? When is Y1 zero?

Return to Y=. Enter the equation y=abs(x)+7 into Y2 using [MATH] abs(Examine the graph. What seems to be the relationship between the graphs?

Examine the table. Is the relationship between Y2 and Y1 what you were expecting? Why or why not? Where are the Y values equal to zero?

Return to Y=. Enter the equation y=abs(x+7) into Y2 as shown.

Examine the graph. What seems to be the relationship between the graphs? How is this picture different from the graph with y=abs(x)+7?

Examine the table. Is the relationship between Y2 and Y1 what you were expecting? Why or why not? Where are the Y values equal to zero?

Compare y=abs(x)+7 to y=abs(x+7). How are they similar? Different?

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TI.MAT.ENG.SE.1.Algebra-I.7.2

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