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Related Lessons: Solving Absolute Value Equations Solving Absolute Value Inequalities

 

Graph of Absolute Value Functions

 

This lesson is about graphing an absolute value function when the expression inside the absolute value symbol is linear. It is linear if the variable "x" has a power of 1. The graph of absolute value function has a shape of "V" or inverted "V".

The general form of an absolute value function that is linear is

 

y=|mx+b|+c
  • where the vertex (low or high point) is located at (-b/m , c )
  • the vertical line x=-b/m divides the graph into two equal halves

 

The specific approach that we are going to use is to find the "right amount of points" that can be plotted on the xy-axis so that we have a good approximation on how the graph of the absolute value function looks likes.

Don't fall into the common trap to always use x = 0 as the middle value of the x coordinates. This may work at times, but this idea is not so reliable.

I suggest to use the x-coordinate of the vertex , (-b/m , c), which is x=-b/m as the middle value of all x-values in the table. In other words, once you have determined what the middle value for x is, then go ahead and pick any arbitrary x values to the left and right of it as long as it is properly spaced out.

Again, the best way to illustrate this simple idea is through the use of examples!

 


 

Example 1: Graph the absolute value function y=|x| using table of values.

This is the most basic form of an absolute value function. If you see that the only expression inside the absolute value symbol is just "x", assume that the vertex of the graph will occur when x = 0.

Or, if you want to use the formula above to find the vertex, rewrite the given function in the standard form so that you can identify the values of m, b and c.

y=|x| implies y=|1x+0|+0
  • where m=1, b=0 and c=0

The vertex is calculated as

vertex = (0,0)

 

Since x = 0, this becomes the center values of all x's. Now we can pick some numbers to the left and to the right of zero. I would suggest to use equal amounts of numbers that are of the same increment.

Pick -3, -2, -1 to the left of zero, and +1, +2, +3 to its right. Evaluate all values of x into the function y=|x| to get the corresponding y-values.

 

table of values of y=|x| where x = -3, -2, -1, 0, 1, 2, 3

 

Notice that the graph has a low point determined by the middle x-value which is the x-coordinate of the vertex itself, i.e. (0,0).

 

graph of absolute value function y=|x| where the vertext is at (0,0)

 


 

Example 2: Graph the absolute value function y=|x-2| using table of values.

 

The first step is to find the x-coordinate of the vertex which will serve as the center point in the table of values of x.

Rewrite y=|x-2| as y=|1x+(-2)|+0 where m=1, b=-2 and c=0. We calculate the vertex as...

vertex is calculated as (2,0)

 

The x-coordinate of the vertex will be the center value of all x's in the table. We generated the rest of the x by finding three numbers to the left and right of the middle value of 2 with an increment of 1. You may use an increment of 2, and trust me, the graph will be the same.

 

table of values of y=|x-2| where x=-1, 0, 1, 2, 3, 4 , 5

 

Plot the points on the xy-plane and connect the dots with a straight edge. If you get it right, you should have something similar below. As you can see, the low point of the graph is the vertex located at (2,0).

 

graph of y=|x-2| with vertex at (2,0)

 


Example 3: Graph the absolute value function y=|2x+6|-4 using table of values.

I hope you start realizing that the first step is to always express the given absolute value function in standard form. This allows us to identify the correct values of m, b and c which we will use to substitute into the formula.

y=|2x+6|-4 is written as y=|2x+(6)|+(-4)

Clearly, the required values are m=2, b=6, and c=-4.  Then we calculate the vertex as follows;

vertex = (-3,-4)

Our table of values will have a center value of x = −3. Generate 3 numbers both to the left and right of x = −3 with an increment of 1. Then evaluate each value of x into the function y=|2x+6|-4 to get the corresponding values of y in the table.

Your table should look something like this...

table of values of y=|x-2| where x=-6, -5, -4, -3, -2, -1, 0

 Plot the points on the Cartesian plane and connect them using a straight edge such as a ruler.

graph of y=|2x+6|-4  with a vertex of (-3,-4)

 


 

Example 4: Graph the absolute value function y=-|x-3|-2 using table of values.

This is an example of an absolute value function whose graph is an inverted "V". This happens because the coefficient of the absolute value symbol is negative, that is, −1 .

Let's rewrite this in the standard form.

 

y=-1*|1*x+(-3)|+(-2)

That means m=1, b=-3 and c=-2.

 

Solving the vertex of the function,

vertex of y=-|x-3|-2 is (3,-2)

 

The table of values will have a center value of x = 3.

table of values of y=-|x-3|-2 with x-values of x=0,1,2,3,4,5,6

 

Plotting the points in the xy-axis,

 

graph of the absolute value function y=-|x-3|-2

 


 

Practice Problems with Answers
Worksheet 1 Worksheet 2

 

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