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# Characteristics of Graphs of Exponential Functions

### Learning Outcomes

• Determine whether an exponential function and its associated graph represents growth or decay.
• Sketch a graph of an exponential function.
Before we begin graphing, it is helpful to review the behavior of exponential growth. Recall the table of values for a function of the form $f\left(x\right)={b}^{x}$ whose base is greater than one. We’ll use the function $f\left(x\right)={2}^{x}$. Observe how the output values in the table below change as the input increases by 1.
 x –3 –2 –1 0 1 2 3 $f\left(x\right)={2}^{x}$ $\frac{1}{8}$ $\frac{1}{4}$ $\frac{1}{2}$ 1 2 4 8
Each output value is the product of the previous output and the base, 2. We call the base 2 the constant ratio. In fact, for any exponential function with the form $f\left(x\right)=a{b}^{x}$, b is the constant ratio of the function. This means that as the input increases by 1, the output value will be the product of the base and the previous output, regardless of the value of a. Notice from the table that:
• the output values are positive for all values of x
• as x increases, the output values increase without bound
• as x decreases, the output values grow smaller, approaching zero
The graph below shows the exponential growth function $f\left(x\right)={2}^{x}$.
Notice that the graph gets close to the x-axis but never touches it.
The domain of $f\left(x\right)={2}^{x}$ is all real numbers, the range is $\left(0,\infty \right)$, and the horizontal asymptote is $y=0$. To get a sense of the behavior of exponential decay, we can create a table of values for a function of the form $f\left(x\right)={b}^{x}$ whose base is between zero and one. We’ll use the function $g\left(x\right)={\left(\frac{1}{2}\right)}^{x}$. Observe how the output values in the table below change as the input increases by 1.
 x –3 –2 –1 0 1 2 3 $g\left(x\right)=\left(\frac{1}{2}\right)^{x}$ 8 4 2 1 $\frac{1}{2}$ $\frac{1}{4}$ $\frac{1}{8}$
Again, because the input is increasing by 1, each output value is the product of the previous output and the base or constant ratio $\frac{1}{2}$. Notice from the table that:
• the output values are positive for all values of x
• as x increases, the output values grow smaller, approaching zero
• as x decreases, the output values grow without bound
The graph below shows the exponential decay function, $g\left(x\right)={\left(\frac{1}{2}\right)}^{x}$.
The domain of $g\left(x\right)={\left(\frac{1}{2}\right)}^{x}$ is all real numbers, the range is $\left(0,\infty \right)$, and the horizontal asymptote is $y=0$.

### A General Note: Characteristics of the Graph of the Parent Function $f\left(x\right)={b}^{x}$

An exponential function with the form $f\left(x\right)={b}^{x}$, $b>0$, $b\ne 1$, has these characteristics:
• one-to-one function
• horizontal asymptote: $y=0$
• domain: $\left(-\infty , \infty \right)$
• range: $\left(0,\infty \right)$
• x-intercept: none
• y-intercept: $\left(0,1\right)$
• increasing if $b>1$
• decreasing if $b<1$
Use an online graphing tool to graph $f\left(x\right)={b}^{x}$. Adjust the $b$ values to see various graphs of exponential growth and decay functions. Try decimal values between $0$ and $1$, and also values greater than $1$. Which one is growth and which one is decay?

### How To: Given an exponential function of the form $f\left(x\right)={b}^{x}$, graph the function

1. Create a table of points.
2. Plot at least 3 point from the table including the y-intercept $\left(0,1\right)$.
3. Draw a smooth curve through the points.
4. State the domain, $\left(-\infty ,\infty \right)$, the range, $\left(0,\infty \right)$, and the horizontal asymptote, $y=0$.

### tip for success

When sketching the graph of an exponential function by plotting points, include a few input values left and right of zero as well as zero itself. With few exceptions, such as functions that would be undefined at zero or negative input like the radical or (as you'll see soon) the logarithmic function, it is good practice to let the input equal $-3, -2, -1, 0, 1, 2, \text{ and } 3$ to get the idea of the shape of the graph.

### Example: Sketching the Graph of an Exponential Function of the Form $f\left(x\right)={b}^{x}$

Sketch a graph of $f\left(x\right)={0.25}^{x}$. State the domain, range, and asymptote.

Answer: Before graphing, identify the behavior and create a table of points for the graph.

• Since = 0.25 is between zero and one, we know the function is decreasing. The left tail of the graph will increase without bound, and the right tail will approach the asymptote = 0.
• Create a table of points.  x –3 –2 –1 0 1 2 3 $f\left(x\right)={0.25}^{x}$ 64 16 4 1 0.25 0.0625 0.015625
• Plot the y-intercept, $\left(0,1\right)$, along with two other points. We can use $\left(-1,4\right)$ and $\left(1,0.25\right)$.
Draw a smooth curve connecting the points.
The domain is $\left(-\infty ,\infty \right)$, the range is $\left(0,\infty \right)$, and the horizontal asymptote is $y=0$.

Watch the following video for another example of graphing an exponential function. The base of the exponential term is between $0$ and $1$, so this graph will represent decay. https://youtu.be/FMzZB9Ve-1U The next example shows how to plot an exponential growth function where the base is greater than $1$.

### Example

Sketch a graph of $f(x)={\sqrt{2}(\sqrt{2})}^{x}$. State the domain and range.

Before graphing, identify the behavior and create a table of points for the graph.

• Since = $\sqrt{2}$, which is greater than one, we know the function is increasing, and we can verify this by creating a table of values. The left tail of the graph will get really close to the x-axis and the right tail will increase without bound.
• Create a table of points.  x $–3$ $–2$ $–1$ $0$ $1$ $2$ $3$ $f\left(x\right)=\sqrt{2}{(\sqrt{2})}^{x}$ $0.5$ $0.71$ $1$ $1.41$ $2$ $2.83$ $4$
• Plot the y-intercept, $\left(0,1.41\right)$, along with two other points. We can use $\left(-1,1\right)$ and $\left(1,2\right)$.

Draw a smooth curve connecting the points.

The domain is $\left(-\infty ,\infty \right)$; the range is $\left(0,\infty \right)$.

The next video example includes graphing an exponential growth function and defining the domain and range of the function. https://youtu.be/M6bpp0BRIf0

### Try It

Sketch the graph of $f\left(x\right)={4}^{x}$. State the domain, range, and asymptote.

Answer: The domain is $\left(-\infty ,\infty \right)$, the range is $\left(0,\infty \right)$, and the horizontal asymptote is $y=0$.

[ohm_question]3607[/ohm_question]