ORCCA Using Technology to Explore Functions

Section10.3Using Technology to Explore Functions

ObjectivesPCC Course Content and Outcome Guide

C.4.1:1

Graphing technology allows us to explore the properties of functions more deeply than we can with only pencil and paper. It can quickly create a table of values, and quickly plot the graph of a function. Such technology can also evaluate functions, solve equations with functions, find maximum and minimum values, and explore other key features.

There are many graphing technologies currently available, including (but not limited to) physical (hand-held) graphing calculators, Desmos, GeoGebra, Sage, and WolframAlpha.

This section will focus on how technology can be used to explore functions and their key features. Although the choice of particular graphing technology varies by each school and curriculum, the main ways in which technology is used to explore functions is the same and can be done with each of the technologies above.

Figure10.3.1Alternative Video Lesson

Subsection10.3.1Finding an Appropriate Window

With a simple linear equation like $y=2x+5\text{,}$ most graphing technologies will show this graph in a good window by default. A common default window goes from $x=-10$ to $x=10$ and $y=-10$ to $y=10\text{.}$

What if we wanted to graph something with a much larger magnitude though, such as $y=2000x+5000\text{?}$ If we tried to view this for $x=-10$ to $x=10$ and $y=-10$ to $y=10\text{,}$ the function would appear as an almost vertical line since it has such a steep slope.

Using technology, we will create a table of values for this function as shown in Figure 10.3.2.(a). Then we will to set the $x$ -values for which we view the function to go from $x=-5$ to $x=5$ and the $y$ -values from $y=-20{,}000$ to $y=20{,}000\text{.}$ The graph is shown in Figure 10.3.2.(b).

$x$	$y=2000x+5000$
$-5$	$-5000$
$-4$	$-3000$
$-3$	$-1000$
$-2$	$1000$
$-1$	$3000$
$0$	$5000$
$1$	$7000$
$2$	$9000$
$3$	$11000$
$4$	$13000$
$5$	$15000$

(a)A table of values

(b)Graphed with an appropriate window

Figure10.3.2Creating a table of values to determine an appropriate graphing window

Now let's practice finding an appropriate viewing window with a less familiar function.

Example10.3.3

Find an appropriate window for $q(x)=\frac{x^3}{100}-2x+1\text{.}$

Entering this function into graphing technology, we input q(x)=(x^3)/100-2x+1. A default window will generally give us something like this:

Figure10.3.4Function

$q$ graphed in the default window.

We can tell from the lower right corner of Figure 10.3.4 that we're not quite viewing all of the important details of this function. To determine a better window, we could use technology to make a table of values. Another more rudimentary option is to double the viewing constraints for $x$ and $y\text{,}$ as shown in Figure 10.3.5. Many graphing technologies have the ability to zoom in and out quickly.

Figure10.3.5Function

$q$ graphed in an expanded window.

Subsection10.3.2Using Technology to Determine Key Features of a Graph

The key features of a graph can be determined using graphing technology. Here, we'll show how to determine the $x$ -intercepts, $y$ -intercepts, maximum/minimum values, and the domain and range using technology.

Example10.3.6

Graph the function given by $p(x)=-1000x^2-100x+40\text{.}$ Determine an appropriate viewing window, and then use graphing technology to determine the following:

Determine the $x$ -intercepts of the function.
Determine the $y$ -intercept of the function.
Determine the maximum function value and where it occurs.
State the domain and range of this function.

Solution

To start, we'll take a quick view of this function in a default window. We can see that we need to zoom in on the $x$-values, but we need to zoom out on the $y$-values.

From the graph we see that the $x$-values might as well run from about $-0.5$ to $0.5\text{,}$ so we will look at $x$-values in that window in increments of $0.1\text{,}$ as shown in Table 10.3.8.(a). This table allows us to determine an appropriate viewing window for $y=p(x)$ which is shown in Figure 10.3.8.(b). The table suggests we should go a little higher than $40$ on the $y$-axis, and it would be OK to go the same distance in the negative direction to keep the $x$-axis centered.

Figure10.3.7Graph of $y=p(x)$ in an inappropriate window

$x$	$p(x)$
$-0.5$	$-160$
$-0.4$	$-80$
$-0.3$	$-20$
$-0.2$	$20$
$-0.1$	$40$
$0$	$40$
$0.1$	$20$
$0.2$	$-20$
$0.3$	$-80$
$0.4$	$-160$
$0.5$	$-260$

(a)Function values for $y=p(x)$

(b)Graph of $y=p(x)$ in an appropriate window showing key features

Figure10.3.8Creating a table of values to determine an appropriate graphing window

We can now use Figure 10.3.8.(b) to determine the $x$-intercepts, the $y$-intercept, the maximum function value, and the domain and range.

To determine the $x$-intercepts, we will find the points where $y$ is zero. These are about $(-0.2562,0)$ and $(0.1562,0)\text{.}$
To determine the $y$-intercept, we need the point where $x$ is zero. This point is $(0,40)\text{.}$
The highest point on the graph is the vertex, which is about $(-0.05,42.5)\text{.}$ So the maximum function value is $42.5$ and occurs at $-0.05\text{.}$
We can see that the function is defined for all $x$-values, so the domain is $(-\infty,\infty)\text{.}$ The maximum function value is $42.5\text{,}$ and there is no minimum function value. Thus the range is $(-\infty,42.5]\text{.}$

Example10.3.9

If we use graphing technology to graph the function $g$ where $g(x) = 0.0002x^2 + 0.00146x + 0.00266\text{,}$ we may be mislead by the way values are rounded. Without technology, we know that this function is a quadratic function and therefore has at most two $x$ -intercepts and has a vertex that will determine the minimum function value. However, using technology we could obtain a graph with the following key points:

Figure10.3.10Misleading graph

This looks like there are three $x$ -intercepts, which we know is not possible for a quadratic function. We can evaluate $g$ at $x=-3.65$ and determine that $g(-3.65)= -0.0000045\text{,}$ which is approximately zero when rounded. So the true vertex of this function is $(-3.65,-0.0000045)\text{,}$ and the minimum value of this function is $-0.0000045$ (not zero).

Every graphing tool generally has some type of limitation like this one, and it's good to be aware that these limitations exist.

Subsection10.3.3Solving Equations and Inequalities Graphically Using Technology

To algebraically solve an equation like $h(x)=v(x)$ for

$\begin{align*} h(x)\amp=-0.01(x-90)(x+20)\amp\amp\text{and}\amp v(x)\amp=-0.04(x-10)(x-80)\text{,} \end{align*}$

we'd start by setting up

$\begin{equation*} -0.01(x-90)(x+20)=-0.04(x-10)(x-80) \end{equation*}$

To solve this, we'd then simplify each side of the equation, set it equal to zero, and finally use the quadratic formula.

An alternative is to graphically solve this equation, which is done by graphing

$\begin{align*} y\amp=-0.01(x-90)(x+20)\amp\amp\text{and}\amp y\amp=-0.04(x-10)(x-80)\text{.} \end{align*}$

The points of intersection, $(22.46,28.677)$ and $(74.207,14.878)\text{,}$ show where these functions are equal. This means that the $x$ -values give the solutions to the equation $-0.01(x-90)(x+20)=-0.04(x-10)(x-80)\text{.}$ So the solutions are approximately $22.46$ and $74.207\text{,}$ and the solution set is approximately $\{22.46, 74.207\}\text{.}$

Figure10.3.11Points of intersection for

$h(x)=v(x)$

Similarly, to graphically solve an equation like $h(x)=25$ for

$\begin{equation*} h(x)=-0.01(x-90)(x+20)\text{,} \end{equation*}$

we can graph

$\begin{equation*} y=-0.01(x-90)(x+20)\qquad\text{and}\qquad y=25 \end{equation*}$

The points of intersection are $(12.807,25)$ and $(57.913,25)\text{,}$ which tells us that the solutions to $h(x)=25$ are approximately $12.807$ and $57.913\text{.}$ The solution set is approximately $\{12.807, 57.913\}\text{.}$

Figure10.3.12Points of intersection for

$h(x)=25$

Example10.3.13

Use graphing technology to solve the following inequalities:

$-20t^2-70t+300 \geq -5t+300$
$-20t^2-70t+300 \lt -5t+300$

Solution

To solve these inequalities graphically, we will start by graphing the equations $y=-20t^2-70t+300$ and $y=-5t+300$ and determining the points of intersection:

Figure10.3.14Points of intersection for $y=-20t^2-70t+300$ and $y=-5t+300$

To solve $-20t^2-70t+300 \geq -5t+300\text{,}$ we need to determine where the $y$-values of the graph of $y=-20t^2-70t+300$ are greater than the $y$-values of the graph of $y=-5t+300$ in addition to the values where the $y$-values are equal. This region is highlighted in Figure 10.3.15.

Figure10.3.15
We can see that this region includes all values of $t$ between, and including, $t=-3.25$ and $t=0\text{.}$ So the solutions to this inequality include all values of $t$ for which $-3.25\le t \le 0\text{.}$ We can write this solution set in interval notation as $[-3.25,0]$ or in set-builder notation as $\{t\mid -3.25 \leq t \leq 0\}\text{.}$
To now solve $-20t^2-70t+300 \lt -5t+300\text{,}$ we will need to determine where the $y$-values of the graph of $y=-20t^2-70t+300$ are less than the $y$-values of the graph of $y=-5t+300\text{.}$ This region is highlighted in Figure 10.3.16.

Figure10.3.16
We can see that $-20t^2-70t+300 \lt -5t+300$ for all values of $t$ where $t\lt -3.25$ or $t\gt 0\text{.}$ We can write this solution set in interval notation as $(-\infty,-3.25)\cup(0,\infty)$ or in set-builder notation as $\{t\mid t\lt -3.25 \text{ or } t\gt 0\}\text{.}$

SubsectionExercises

Using Technology to Create a Table of Function Values

1

Use technology to make a table of values for the function $F$ defined by $F(x)={x^{2}-3x+5}\text{.}$

$x$	$F(x)$

2

Use technology to make a table of values for the function $G$ defined by $G(x)={-2x^{2}+4x+2}\text{.}$

$x$	$G(x)$

3

Use technology to make a table of values for the function $H$ defined by $H(x)={3.6x^{2}-200x-27}\text{.}$

$x$	$H(x)$

4

Use technology to make a table of values for the function $K$ defined by $K(x)={0.8x^{2}-30x-83}\text{.}$

$x$	$K(x)$

5

Use technology to make a table of values for the function $K$ defined by $K(x)={-6x^{3}+140x+62}\text{.}$

$x$	$K(x)$

6

Use technology to make a table of values for the function $f$ defined by $f(x)={10x^{3}-100x+7}\text{.}$

$x$	$f(x)$

Determining Appropriate Windows

7

Choose an appropriate window for graphing the function $f$ defined by $f(x)={2785x-7082}$ that shows its key features.