homework 7 graphing exponential functions

6.2 Graphs of Exponential Functions

Learning objectives.

• Graph exponential functions.
• Graph exponential functions using transformations.

As we discussed in the previous section, exponential functions are used for many real-world applications such as finance, forensics, computer science, and most of the life sciences. Working with an equation that describes a real-world situation gives us a method for making predictions. Most of the time, however, the equation itself is not enough. We learn a lot about things by seeing their pictorial representations, and that is exactly why graphing exponential equations is a powerful tool. It gives us another layer of insight for predicting future events.

Graphing Exponential Functions

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 ( x ) = b x f ( x ) = b x whose base is greater than one. We’ll use the function f ( x ) = 2 x . f ( x ) = 2 x . Observe how the output values in Table 1 change as the input increases by 1. 1.

Each output value is the product of the previous output and the base, 2. 2. We call the base 2 2 the constant ratio . In fact, for any exponential function with the form f ( x ) = a b x , f ( x ) = a b x , b 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 . a .

Notice from the table that

• the output values are positive for all values of x ; x ;
• as x x increases, the output values increase without bound; and
• as x x decreases, the output values grow smaller, approaching zero.

Figure 1 shows the exponential growth function f ( x ) = 2 x . f ( x ) = 2 x .

The domain of f ( x ) = 2 x f ( x ) = 2 x is all real numbers, the range is ( 0 , ∞ ) , ( 0 , ∞ ) , and the horizontal asymptote is y = 0. 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 ( x ) = b x f ( x ) = b x whose base is between zero and one. We’ll use the function g ( x ) = ( 1 2 ) x . g ( x ) = ( 1 2 ) x . Observe how the output values in Table 2 change as the input increases by 1. 1.

Again, because the input is increasing by 1, each output value is the product of the previous output and the base, or constant ratio 1 2 . 1 2 .

• as x x increases, the output values grow smaller, approaching zero; and
• as x x decreases, the output values grow without bound.

Figure 2 shows the exponential decay function, g ( x ) = ( 1 2 ) x . g ( x ) = ( 1 2 ) x .

The domain of g ( x ) = ( 1 2 ) x g ( x ) = ( 1 2 ) x is all real numbers, the range is ( 0 , ∞ ) , ( 0 , ∞ ) , and the horizontal asymptote is y = 0. y = 0.

Characteristics of the Graph of the Parent Function f ( x ) = b x f ( x ) = b x

An exponential function with the form f ( x ) = b x , f ( x ) = b x , b > 0 , b > 0 , b ≠ 1 , b ≠ 1 , has these characteristics:

• one-to-one function
• horizontal asymptote: y = 0 y = 0
• domain: ( – ∞ ,   ∞ ) ( – ∞ ,   ∞ )
• range: ( 0 , ∞ ) ( 0 , ∞ )
• x- intercept: none
• y- intercept: ( 0 , 1 ) ( 0 , 1 )
• increasing if b > 1 b > 1
• decreasing if b < 1 b < 1

Figure 3 compares the graphs of exponential growth and decay functions.

Given an exponential function of the form f ( x ) = b x , f ( x ) = b x , graph the function.

• Create a table of points.
• Plot at least 3 3 point from the table, including the y -intercept ( 0 , 1 ) . ( 0 , 1 ) .
• Draw a smooth curve through the points.
• State the domain, ( − ∞ , ∞ ) , ( − ∞ , ∞ ) , the range, ( 0 , ∞ ) , ( 0 , ∞ ) , and the horizontal asymptote, y = 0. y = 0.

Sketching the Graph of an Exponential Function of the Form f ( x ) = b x

Sketch a graph of f ( x ) = 0.25 x . f ( x ) = 0.25 x . State the domain, range, and asymptote.

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

• Since b = 0.25 b = 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 y = 0. y = 0.
• Plot the y -intercept, ( 0 , 1 ) , ( 0 , 1 ) , along with two other points. We can use ( − 1 , 4 ) ( − 1 , 4 ) and ( 1 , 0.25 ) . ( 1 , 0.25 ) .

Draw a smooth curve connecting the points as in Figure 4 .

The domain is ( − ∞ , ∞ ) ; ( − ∞ , ∞ ) ; the range is ( 0 , ∞ ) ; ( 0 , ∞ ) ; the horizontal asymptote is y = 0. y = 0.

Sketch the graph of f ( x ) = 4 x . f ( x ) = 4 x . State the domain, range, and asymptote.

Graphing Transformations of Exponential Functions

Transformations of exponential graphs behave similarly to those of other functions. Just as with other parent functions, we can apply the four types of transformations—shifts, reflections, stretches, and compressions—to the parent function f ( x ) = b x f ( x ) = b x without loss of shape. For instance, just as the quadratic function maintains its parabolic shape when shifted, reflected, stretched, or compressed, the exponential function also maintains its general shape regardless of the transformations applied.

Graphing a Vertical Shift

The first transformation occurs when we add a constant d d to the parent function f ( x ) = b x , f ( x ) = b x , giving us a vertical shift d d units in the same direction as the sign. For example, if we begin by graphing a parent function, f ( x ) = 2 x , f ( x ) = 2 x , we can then graph two vertical shifts alongside it, using d = 3 : d = 3 : the upward shift, g ( x ) = 2 x + 3 g ( x ) = 2 x + 3 and the downward shift, h ( x ) = 2 x − 3. h ( x ) = 2 x − 3. Both vertical shifts are shown in Figure 5 .

Observe the results of shifting f ( x ) = 2 x f ( x ) = 2 x vertically:

• The domain, ( − ∞ , ∞ ) ( − ∞ , ∞ ) remains unchanged.
• The y- intercept shifts up 3 3 units to ( 0 , 4 ) . ( 0 , 4 ) .
• The asymptote shifts up 3 3 units to y = 3. y = 3.
• The range becomes ( 3 , ∞ ) . ( 3 , ∞ ) .
• The y- intercept shifts down 3 3 units to ( 0 , − 2 ) . ( 0 , − 2 ) .
• The asymptote also shifts down 3 3 units to y = − 3. y = − 3.
• The range becomes ( − 3 , ∞ ) . ( − 3 , ∞ ) .

Graphing a Horizontal Shift

The next transformation occurs when we add a constant c c to the input of the parent function f ( x ) = b x , f ( x ) = b x , giving us a horizontal shift c c units in the opposite direction of the sign. For example, if we begin by graphing the parent function f ( x ) = 2 x , f ( x ) = 2 x , we can then graph two horizontal shifts alongside it, using c = 3 : c = 3 : the shift left, g ( x ) = 2 x + 3 , g ( x ) = 2 x + 3 , and the shift right, h ( x ) = 2 x − 3 . h ( x ) = 2 x − 3 . Both horizontal shifts are shown in Figure 6 .

Observe the results of shifting f ( x ) = 2 x f ( x ) = 2 x horizontally:

• The domain, ( − ∞ , ∞ ) , ( − ∞ , ∞ ) , remains unchanged.
• The asymptote, y = 0 , y = 0 , remains unchanged.
• When the function is shifted left 3 3 units to g ( x ) = 2 x + 3 , g ( x ) = 2 x + 3 , the y -intercept becomes ( 0 , 8 ) . ( 0 , 8 ) . This is because 2 x + 3 = ( 8 ) 2 x , 2 x + 3 = ( 8 ) 2 x , so the initial value of the function is 8. 8.
• When the function is shifted right 3 3 units to h ( x ) = 2 x − 3 , h ( x ) = 2 x − 3 , the y -intercept becomes ( 0 , 1 8 ) . ( 0 , 1 8 ) . Again, see that 2 x − 3 = ( 1 8 ) 2 x , 2 x − 3 = ( 1 8 ) 2 x , so the initial value of the function is 1 8 . 1 8 .

Shifts of the Parent Function f ( x ) = b x

For any constants c c and d , d , the function f ( x ) = b x + c + d f ( x ) = b x + c + d shifts the parent function f ( x ) = b x f ( x ) = b x

• vertically d d units, in the same direction of the sign of d . d .
• horizontally c c units, in the opposite direction of the sign of c . c .
• The y -intercept becomes ( 0 , b c + d ) . ( 0 , b c + d ) .
• The horizontal asymptote becomes y = d . y = d .
• The range becomes ( d , ∞ ) . ( d , ∞ ) .

Given an exponential function with the form f ( x ) = b x + c + d , f ( x ) = b x + c + d , graph the translation.

• Draw the horizontal asymptote y = d . y = d .
• Identify the shift as ( − c , d ) . ( − c , d ) . Shift the graph of f ( x ) = b x f ( x ) = b x left c c units if c c is positive, and right c c units if c c is negative.
• Shift the graph of f ( x ) = b x f ( x ) = b x up d d units if d d is positive, and down d d units if d d is negative.
• State the domain, ( − ∞ , ∞ ) , ( − ∞ , ∞ ) , the range, ( d , ∞ ) , ( d , ∞ ) , and the horizontal asymptote y = d . y = d .

Graphing a Shift of an Exponential Function

Graph f ( x ) = 2 x + 1 − 3. f ( x ) = 2 x + 1 − 3. State the domain, range, and asymptote.

We have an exponential equation of the form f ( x ) = b x + c + d , f ( x ) = b x + c + d , with b = 2 , b = 2 , c = 1 , c = 1 , and d = − 3. d = − 3.

Draw the horizontal asymptote y = d y = d , so draw y = −3. y = −3.

Identify the shift as ( − c , d ) , ( − c , d ) , so the shift is ( − 1 , −3 ) . ( − 1 , −3 ) .

Shift the graph of f ( x ) = b x f ( x ) = b x left 1 units and down 3 units.

The domain is ( − ∞ , ∞ ) ; ( − ∞ , ∞ ) ; the range is ( − 3 , ∞ ) ; ( − 3 , ∞ ) ; the horizontal asymptote is y = −3. y = −3.

Graph f ( x ) = 2 x − 1 + 3. f ( x ) = 2 x − 1 + 3. State domain, range, and asymptote.

Given an equation of the form f ( x ) = b x + c + d f ( x ) = b x + c + d for x , x , use a graphing calculator to approximate the solution.

• Press [Y=] . Enter the given exponential equation in the line headed “ Y 1 = ”.
• Enter the given value for f ( x ) f ( x ) in the line headed “ Y 2 = ”.
• Press [WINDOW] . Adjust the y -axis so that it includes the value entered for “ Y 2 = ”.
• Press [GRAPH] to observe the graph of the exponential function along with the line for the specified value of f ( x ) . f ( x ) .
• To find the value of x , x , we compute the point of intersection. Press [2ND] then [CALC] . Select “intersect” and press [ENTER] three times. The point of intersection gives the value of x for the indicated value of the function.

Approximating the Solution of an Exponential Equation

Solve 42 = 1.2 ( 5 ) x + 2.8 42 = 1.2 ( 5 ) x + 2.8 graphically. Round to the nearest thousandth.

Press [Y=] and enter 1.2 ( 5 ) x + 2.8 1.2 ( 5 ) x + 2.8 next to Y 1 =. Then enter 42 next to Y2= . For a window, use the values –3 to 3 for x x and –5 to 55 for y . y . Press [GRAPH] . The graphs should intersect somewhere near x = 2. x = 2.

For a better approximation, press [2ND] then [CALC] . Select [5: intersect] and press [ENTER] three times. The x -coordinate of the point of intersection is displayed as 2.1661943. (Your answer may be different if you use a different window or use a different value for Guess? ) To the nearest thousandth, x ≈ 2.166. x ≈ 2.166.

Solve 4 = 7.85 ( 1.15 ) x − 2.27 4 = 7.85 ( 1.15 ) x − 2.27 graphically. Round to the nearest thousandth.

Graphing a Stretch or Compression

While horizontal and vertical shifts involve adding constants to the input or to the function itself, a stretch or compression occurs when we multiply the parent function f ( x ) = b x f ( x ) = b x by a constant | a | > 0. | a | > 0. For example, if we begin by graphing the parent function f ( x ) = 2 x , f ( x ) = 2 x , we can then graph the stretch, using a = 3 , a = 3 , to get g ( x ) = 3 ( 2 ) x g ( x ) = 3 ( 2 ) x as shown on the left in Figure 8 , and the compression, using a = 1 3 , a = 1 3 , to get h ( x ) = 1 3 ( 2 ) x h ( x ) = 1 3 ( 2 ) x as shown on the right in Figure 8 .

Stretches and Compressions of the Parent Function f ( x ) = b x f ( x ) = b x

For any factor a > 0 , a > 0 , the function f ( x ) = a ( b ) x f ( x ) = a ( b ) x

• is stretched vertically by a factor of a a if | a | > 1. | a | > 1.
• is compressed vertically by a factor of a a if | a | < 1. | a | < 1.
• has a y -intercept of ( 0 , a ) . ( 0 , a ) .
• has a horizontal asymptote at y = 0 , y = 0 , a range of ( 0 , ∞ ) , ( 0 , ∞ ) , and a domain of ( − ∞ , ∞ ) , ( − ∞ , ∞ ) , which are unchanged from the parent function.

Graphing the Stretch of an Exponential Function

Sketch a graph of f ( x ) = 4 ( 1 2 ) x . f ( x ) = 4 ( 1 2 ) x . State the domain, range, and asymptote.

Before graphing, identify the behavior and key points on the graph.

• Since b = 1 2 b = 1 2 is between zero and one, the left tail of the graph will increase without bound as x x decreases, and the right tail will approach the x -axis as x x increases.
• Since a = 4 , a = 4 , the graph of f ( x ) = ( 1 2 ) x f ( x ) = ( 1 2 ) x will be stretched by a factor of 4. 4.
• Plot the y- intercept, ( 0 , 4 ) , ( 0 , 4 ) , along with two other points. We can use ( − 1 , 8 ) ( − 1 , 8 ) and ( 1 , 2 ) . ( 1 , 2 ) .

Draw a smooth curve connecting the points, as shown in Figure 9 .

Sketch the graph of f ( x ) = 1 2 ( 4 ) x . f ( x ) = 1 2 ( 4 ) x . State the domain, range, and asymptote.

Graphing Reflections

In addition to shifting, compressing, and stretching a graph, we can also reflect it about the x -axis or the y -axis. When we multiply the parent function f ( x ) = b x f ( x ) = b x by −1 , −1 , we get a reflection about the x -axis. When we multiply the input by −1 , −1 , we get a reflection about the y -axis. For example, if we begin by graphing the parent function f ( x ) = 2 x , f ( x ) = 2 x , we can then graph the two reflections alongside it. The reflection about the x -axis, g ( x ) = −2 x , g ( x ) = −2 x , is shown on the left side of Figure 10 , and the reflection about the y -axis h ( x ) = 2 − x , h ( x ) = 2 − x , is shown on the right side of Figure 10 .

Reflections of the Parent Function f ( x ) = b x f ( x ) = b x

The function f ( x ) = − b x f ( x ) = − b x

• reflects the parent function f ( x ) = b x f ( x ) = b x about the x -axis.
• has a y -intercept of ( 0 , − 1 ) . ( 0 , − 1 ) .
• has a range of ( − ∞ , 0 ) . ( − ∞ , 0 ) .
• has a horizontal asymptote at y = 0 y = 0 and domain of ( − ∞ , ∞ ) , ( − ∞ , ∞ ) , which are unchanged from the parent function.

The function f ( x ) = b − x f ( x ) = b − x

• reflects the parent function f ( x ) = b x f ( x ) = b x about the y -axis.
• has a y -intercept of ( 0 , 1 ) , ( 0 , 1 ) , a horizontal asymptote at y = 0 , y = 0 , a range of ( 0 , ∞ ) , ( 0 , ∞ ) , and a domain of ( − ∞ , ∞ ) , ( − ∞ , ∞ ) , which are unchanged from the parent function.

Writing and Graphing the Reflection of an Exponential Function

Find and graph the equation for a function, g ( x ) , g ( x ) , that reflects f ( x ) = ( 1 4 ) x f ( x ) = ( 1 4 ) x about the x -axis. State its domain, range, and asymptote.

Since we want to reflect the parent function f ( x ) = ( 1 4 ) x f ( x ) = ( 1 4 ) x about the x- axis, we multiply f ( x ) f ( x ) by − 1 − 1 to get, g ( x ) = − ( 1 4 ) x . g ( x ) = − ( 1 4 ) x . Next we create a table of points as in Table 5 .

Plot the y- intercept, ( 0 , −1 ) , ( 0 , −1 ) , along with two other points. We can use ( −1 , −4 ) ( −1 , −4 ) and ( 1 , −0.25 ) . ( 1 , −0.25 ) .

Draw a smooth curve connecting the points:

The domain is ( − ∞ , ∞ ) ; ( − ∞ , ∞ ) ; the range is ( − ∞ , 0 ) ; ( − ∞ , 0 ) ; the horizontal asymptote is y = 0. y = 0.

Find and graph the equation for a function, g ( x ) , g ( x ) , that reflects f ( x ) = 1.25 x f ( x ) = 1.25 x about the y -axis. State its domain, range, and asymptote.

Summarizing Translations of the Exponential Function

Now that we have worked with each type of translation for the exponential function, we can summarize them in Table 6 to arrive at the general equation for translating exponential functions.

Translations of Exponential Functions

A translation of an exponential function has the form

Where the parent function, y = b x , y = b x , b > 1 , b > 1 , is

• shifted horizontally c c units to the left.
• stretched vertically by a factor of | a | | a | if | a | > 0. | a | > 0.
• compressed vertically by a factor of | a | | a | if 0 < | a | < 1. 0 < | a | < 1.
• shifted vertically d d units.
• reflected about the x- axis when a < 0. a < 0.

Note the order of the shifts, transformations, and reflections follow the order of operations.

Writing a Function from a Description

Write the equation for the function described below. Give the horizontal asymptote, the domain, and the range.

• f ( x ) = e x f ( x ) = e x is vertically stretched by a factor of 2 2 , reflected across the y -axis, and then shifted up 4 4 units.

We want to find an equation of the general form   f ( x ) = a b x + c + d .   f ( x ) = a b x + c + d . We use the description provided to find a , a , b , b , c , c , and d . d .

• We are given the parent function f ( x ) = e x , f ( x ) = e x , so b = e . b = e .
• The function is stretched by a factor of 2 2 , so a = 2. a = 2.
• The function is reflected about the y -axis. We replace x x with − x − x to get: e − x . e − x .
• The graph is shifted vertically 4 units, so d = 4. d = 4.

Substituting in the general form we get,

The domain is ( − ∞ , ∞ ) ; ( − ∞ , ∞ ) ; the range is ( 4 , ∞ ) ; ( 4 , ∞ ) ; the horizontal asymptote is y = 4. y = 4.

Write the equation for function described below. Give the horizontal asymptote, the domain, and the range.

• f ( x ) = e x f ( x ) = e x is compressed vertically by a factor of 1 3 , 1 3 , reflected across the x -axis and then shifted down 2 2 units.

Access this online resource for additional instruction and practice with graphing exponential functions.

• Graph Exponential Functions

6.2 Section Exercises

What role does the horizontal asymptote of an exponential function play in telling us about the end behavior of the graph?

What is the advantage of knowing how to recognize transformations of the graph of a parent function algebraically?

The graph of f ( x ) = 3 x f ( x ) = 3 x is reflected about the y -axis and stretched vertically by a factor of 4. 4. What is the equation of the new function, g ( x ) ? g ( x ) ? State its y -intercept, domain, and range.

The graph of f ( x ) = ( 1 2 ) − x f ( x ) = ( 1 2 ) − x is reflected about the y -axis and compressed vertically by a factor of 1 5 . 1 5 . What is the equation of the new function, g ( x ) ? g ( x ) ? State its y -intercept, domain, and range.

The graph of f ( x ) = 10 x f ( x ) = 10 x is reflected about the x -axis and shifted upward 7 7 units. What is the equation of the new function, g ( x ) ? g ( x ) ? State its y -intercept, domain, and range.

The graph of f ( x ) = ( 1.68 ) x f ( x ) = ( 1.68 ) x is shifted right 3 3 units, stretched vertically by a factor of 2 , 2 , reflected about the x -axis, and then shifted downward 3 3 units. What is the equation of the new function, g ( x ) ? g ( x ) ? State its y -intercept (to the nearest thousandth), domain, and range.

The graph of f x = - 1 2 ( 1 4 ) x - 2 + 4 f x = - 1 2 ( 1 4 ) x - 2 + 4 is shifted downward 4 4 units, and then shifted left 2 2 units, stretched vertically by a factor of 4 , 4 , and reflected about the x -axis. What is the equation of the new function, g ( x ) ? g ( x ) ? State its y -intercept, domain, and range.

For the following exercises, graph the function and its reflection about the y -axis on the same axes, and give the y -intercept.

f ( x ) = 3 ( 1 2 ) x f ( x ) = 3 ( 1 2 ) x

g ( x ) = − 2 ( 0.25 ) x g ( x ) = − 2 ( 0.25 ) x

h ( x ) = 6 ( 1.75 ) − x h ( x ) = 6 ( 1.75 ) − x

For the following exercises, graph each set of functions on the same axes.

f ( x ) = 3 ( 1 4 ) x , f ( x ) = 3 ( 1 4 ) x , g ( x ) = 3 ( 2 ) x , g ( x ) = 3 ( 2 ) x , and h ( x ) = 3 ( 4 ) x h ( x ) = 3 ( 4 ) x

f ( x ) = 1 4 ( 3 ) x , f ( x ) = 1 4 ( 3 ) x , g ( x ) = 2 ( 3 ) x , g ( x ) = 2 ( 3 ) x , and h ( x ) = 4 ( 3 ) x h ( x ) = 4 ( 3 ) x

For the following exercises, match each function with one of the graphs in Figure 12 .

f ( x ) = 2 ( 0.69 ) x f ( x ) = 2 ( 0.69 ) x

f ( x ) = 2 ( 1.28 ) x f ( x ) = 2 ( 1.28 ) x

f ( x ) = 2 ( 0.81 ) x f ( x ) = 2 ( 0.81 ) x

f ( x ) = 4 ( 1.28 ) x f ( x ) = 4 ( 1.28 ) x

f ( x ) = 2 ( 1.59 ) x f ( x ) = 2 ( 1.59 ) x

f ( x ) = 4 ( 0.69 ) x f ( x ) = 4 ( 0.69 ) x

For the following exercises, use the graphs shown in Figure 13 . All have the form f ( x ) = a b x . f ( x ) = a b x .

Which graph has the largest value for b ? b ?

Which graph has the smallest value for b ? b ?

Which graph has the largest value for a ? a ?

Which graph has the smallest value for a ? a ?

For the following exercises, graph the function and its reflection about the x -axis on the same axes.

f ( x ) = 1 2 ( 4 ) x f ( x ) = 1 2 ( 4 ) x

f ( x ) = 3 ( 0.75 ) x − 1 f ( x ) = 3 ( 0.75 ) x − 1

f ( x ) = − 4 ( 2 ) x + 2 f ( x ) = − 4 ( 2 ) x + 2

For the following exercises, graph the transformation of f ( x ) = 2 x . f ( x ) = 2 x . Give the horizontal asymptote, the domain, and the range.

f ( x ) = 2 − x f ( x ) = 2 − x

h ( x ) = 2 x + 3 h ( x ) = 2 x + 3

f ( x ) = 2 x − 2 f ( x ) = 2 x − 2

For the following exercises, describe the end behavior of the graphs of the functions.

f ( x ) = − 5 ( 4 ) x − 1 f ( x ) = − 5 ( 4 ) x − 1

f ( x ) = 3 ( 1 2 ) x − 2 f ( x ) = 3 ( 1 2 ) x − 2

f ( x ) = 3 ( 4 ) − x + 2 f ( x ) = 3 ( 4 ) − x + 2

For the following exercises, start with the graph of f ( x ) = 4 x . f ( x ) = 4 x . Then write a function that results from the given transformation.

Shift f ( x ) f ( x ) 4 units upward

Shift f ( x ) f ( x ) 3 units downward

Shift f ( x ) f ( x ) 2 units left

Shift f ( x ) f ( x ) 5 units right

Reflect f ( x ) f ( x ) about the x -axis

Reflect f ( x ) f ( x ) about the y -axis

For the following exercises, each graph is a transformation of y = 2 x . y = 2 x . Write an equation describing the transformation.

For the following exercises, find an exponential equation for the graph.

For the following exercises, evaluate the exponential functions for the indicated value of x . x .

g ( x ) = 1 3 ( 7 ) x − 2 g ( x ) = 1 3 ( 7 ) x − 2 for g ( 6 ) . g ( 6 ) .

f ( x ) = 4 ( 2 ) x − 1 − 2 f ( x ) = 4 ( 2 ) x − 1 − 2 for f ( 5 ) . f ( 5 ) .

h ( x ) = − 1 2 ( 1 2 ) x + 6 h ( x ) = − 1 2 ( 1 2 ) x + 6 for h ( − 7 ) . h ( − 7 ) .

For the following exercises, use a graphing calculator to approximate the solutions of the equation. Round to the nearest thousandth.

− 50 = − ( 1 2 ) − x − 50 = − ( 1 2 ) − x

116 = 1 4 ( 1 8 ) x 116 = 1 4 ( 1 8 ) x

12 = 2 ( 3 ) x + 1 12 = 2 ( 3 ) x + 1

5 = 3 ( 1 2 ) x − 1 − 2 5 = 3 ( 1 2 ) x − 1 − 2

− 30 = − 4 ( 2 ) x + 2 + 2 − 30 = − 4 ( 2 ) x + 2 + 2

Explore and discuss the graphs of F ( x ) = ( b ) x F ( x ) = ( b ) x and G ( x ) = ( 1 b ) x . G ( x ) = ( 1 b ) x . Then make a conjecture about the relationship between the graphs of the functions b x b x and ( 1 b ) x ( 1 b ) x for any real number b > 0. b > 0.

Prove the conjecture made in the previous exercise.

Explore and discuss the graphs of f ( x ) = 4 x , f ( x ) = 4 x , g ( x ) = 4 x − 2 , g ( x ) = 4 x − 2 , and h ( x ) = ( 1 16 ) 4 x . h ( x ) = ( 1 16 ) 4 x . Then make a conjecture about the relationship between the graphs of the functions b x b x and ( 1 b n ) b x ( 1 b n ) b x for any real number n and real number b > 0. b > 0.

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4.2: Graphs of Exponential Functions

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Learning Objectives

• Graph exponential functions and their transformations.
• Review Laws of Exponents

Exponential functions are used for many real-world applications such as finance, forensics, computer science, and most of the life sciences. Working with an equation that describes a real-world situation gives us a method for making predictions. Seeing their graphs gives us another layer of insight for predicting future events.

Graph Basic Exponential Functions

Exponential growth is modelled by functions of the form \(f(x)=b^x\) where the base is greater than one. Exponential decay occurs when the base is between zero and one. We’ll use the functions \(f(x)=2^x\) and  \(g(x)={\left(\tfrac{1}{2}\right)}^x\) to get some insight into the behaviour of graphs that model exponential growth and decay. In each table of values below, observe how the output values change as the input increases by \(1\).

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 \(f(x)=ab^x,\) \(b\) is the constant ratio of the function. This means that as the input increases by \(1\), the output value \(f(x+1)\) will be the product of the base and the previous output, \(b f(x)=b \cdot ab^x=ab^{x+1}=f(x+1)\), 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 domain of \(f(x)=2^x\) is all real numbers, the range is \((0,\infty)\), and the horizontal asymptote is \(y=0\).

Figure \(\PageIndex{1}\): Graph of the exponential growth function \(f(x)=2^x\). Notice that the graph gets close to the x-axis, but never touches it.

Again, as the input is increases by \(1\), each output value is the product of the previous output and the base, \(\tfrac{1}{2}\), so the constant ratio of the function is \(\tfrac{1}{2}\). 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; and
• as \(x\) decreases, the output values grow without bound.

The domain of \(g\) is all real numbers, the range is \((0,\infty)\), and the horizontal asymptote is \(y=0\).

Figure \(\PageIndex{2}\): Graph of the exponential decay function, \(g(x)={\left(\frac{1}{2}\right)}^x\).

Notice that \(g(x)={\left(\frac{1}{2}\right)}^x = (2^{-1})^x = f(-x)\). Thus the graph of \(g\) is simply a reflection over the \(y\)-axis of the graph of \(f\).

CHARACTERISTICS OF THE GRAPH OF THE PARENT FUNCTION \(f(x) = b^x\)

• Draw and label the horizontal asymptote, \( y=0. \)
• Create a table of points and use it to plot at least \(3\) points, including the \(y\)-intercept \( (0, 1) \) and key point \( (1, b) \).
• Draw a smooth curve that goes through the points and approaches the horizontal asymptote.
• State the domain \((−\infty,\infty)\), the range \((0,\infty)\), and the horizontal asymptote, \( y=0. \)

Example \(\PageIndex{1}\): Sketch the Graph of an Exponential Function of the Form \(f(x) = b^x\)

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

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

• Since \(b=0.25\) 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 \(y=0\).
• Create a table of points as in Table \(\PageIndex{3}\).
• Plot the asymptote, and the y -intercept \((0,1)\), along with two other points. We can use \((−1,4)\) and \((1,0.25)\).

Draw a smooth curve connecting the points as shown to the right.

The domain is \((−\infty,\infty)\); the range is \((0,\infty)\); the horizontal asymptote is \(y=0\).

Graph Transformations of Exponential Functions

Transformations of exponential graphs behave similarly to those of other functions. Just as with other parent functions, we can apply the four types of transformations—shifts, reflections, stretches, and compressions—to the parent function \(f(x)=b^x\) without loss of general shape. For instance, just as the quadratic function maintains its parabolic shape when shifted, reflected, stretched, or compressed, the exponential function also maintains its general shape regardless of the transformations applied.

Vertical Shifts

Observe the results of shifting \(f(x)=2^x\) vertically:

• The domain, \((−\infty,\infty)\) remains unchanged.
• The \(y\) - intercept shifts up \(3\) units to \((0,4)\).
• The asymptote shifts up \(3\) units to \(y=3\).
• The range becomes \((3,\infty)\).
• The \(y\) - intercept shifts down \(3\) units to \((0,−2)\).
• The asymptote also shifts down \(3\) units to \(y=−3\).
• The range becomes \((−3,\infty)\).

Horizontal Shifts

Observe the results of shifting \(f(x)=2^x\) horizontally:

• The domain, \((−\infty,\infty)\), remains unchanged.
• The asymptote, \(y=0\), remains unchanged.
• When the function is shifted left \(3\) units to \(g(x)=2^{x+3}\), the \(y\)-intercept becomes \((0,8)\). This is because \(2^{x+3}=(8)2^x\), so the initial value of the function is \(8\).
• When the function is shifted right \(3\) units to \(h(x)=2^{x−3}\), the \(y\)-intercept becomes \((0,\frac{1}{8})\). Again, see that \(2^{x−3}=(\frac{1}{8})2^x\), so the initial value of the function is \(\frac{1}{8}\).

SHIFTS OF THE PARENT FUNCTION \( y = b^x\)

For any constants \(c\) and \(d\), the function \(f(x)=b^{x+c}+d\) shifts the parent function \(y=b^x\)

• If \(d>0\) the parent function is shifted up \(d\) units
• If \(d<0\) the parent function is shifted down \(d\) units
• The new \(y\)-coordinates are equal to \(y+d\)
• The horizontal asymptote becomes \(y=d\), and occurs when the exponential part of the function, \(b^{x+c}\) approaches zero.
• The range becomes \((d,\infty)\).
• If \(c>0\) the parent function is shifted left  \(c\) units
• If \(c<0\) the parent function is shifted right  \(c\) units
• The new \(x\)-coordinates are equal to \(x-c\).;
• The new  y -intercept, at \((0,1)\) in the parent function, occuring at \(f(0)\), becomes \((0,b^c+d)\).
• Draw the horizontal asymptote \(y=d\).
• Identify the shift as \((−c,d)\). Shift the graph of \(f(x)=b^x\) left \(c\) units if \(c\) is positive, and right \(c\) units if \(c\) is negative.
• Shift the graph of \(f(x)=b^x\) up \(d\) units if \(d\) is positive, and down \(d\) units if \(d\) is negative.
• State the domain, \((−\infty,\infty)\), the range, \((d,\infty)\), and the horizontal asymptote \(y=d\).

Example \(\PageIndex{2}\): Graphing a Shift of an Exponential Function

Reflections

In addition to shifting, compressing, and stretching a graph, we can also reflect it about the \(x\)-axis or the \(y\)-axis. When we multiply the parent function \(f(x)=b^x\)  by \(−1\), we get a reflection about the \(x\)-axis. When we multiply the input by \(−1\), we get a reflection about the \(y\)-axis. For example, if we begin by graphing the parent function \(f(x)=2^x\), we can then graph the two reflections alongside it. The reflection about the \(x\)-axis, \(g(x)=−2^x\), is illustrated below in the graph on the left, and the reflection about the \(y\)-axis \(h(x)=2^{−x}\), is shown in the graph on the right.

REFLECTIONS OF THE PARENT FUNCTION \(f(x) = b^x\)

The function \(f(x)=−b^x\)

• reflects the parent function \(f(x)=b^x\) about the \(x\)-axis.
• has a \(y\)-intercept of \((0,−1)\).
• has a range of \((−\infty,0)\)
• has a horizontal asymptote at \(y=0\) and domain of \((−\infty,\infty)\), which are unchanged from the parent function.

The function \(f(x)=−b^x + d\) has both a vertical shift and reflection about the \(x\)-axis. In this situation, always do the vertical shift LAST .

The function \(f(x)=b^{−x}\)

• reflects the parent function \(f(x)=b^x\) about the \(y\)-axis.
• has a \(y\)-intercept of \((0,1)\), a horizontal asymptote at \(y=0\), a range of \((0,\infty)\), and a domain of \((−\infty,\infty)\), which are unchanged from the parent function.

The function \(f(x)=b^{-x+c}\) has both a horizontal shift and reflection about the \(y\)-axis. In this situation, always do the horizontal shift FIRST .

Example \(\PageIndex{3}\): Construct an Equation for a Reflected Exponential Function

Find and graph the equation for a function,  \(g(x)\), that reflects \(f(x)=( \tfrac{1}{4} )^x\) about the \(x\)-axis. State its domain, range, and asymptote.

 Plot the \(y\) - intercept, \((0,−1)\), along with two other points. We can use \((−1,−4)\) and \((1,−0.25)\). Draw a smooth curve connecting the points. The domain is \((−\infty,\infty)\); the range is \((−\infty,0)\); the horizontal asymptote is \(y=0\). 

Figure \(\PageIndex{11}\). Graph of  \(g(x)=−( \tfrac{1}{4} )^x\).

Find and graph the equation for a function, \(g(x)\), that reflects \(f(x)={1.25}^x\) about the \(y\)-axis. State its domain, range, and asymptote.

The domain is \((−\infty,\infty)\); the range is \((0,\infty)\); the horizontal asymptote is \(y=0\). 

Included in the graph is the horizontal asymptote \(y=0\), and the points for \(g(-1) = 1.25\), \(g(0)=1\), and \(g(1) = .8\).

Vertical Stretches or Compressions

While horizontal and vertical shifts involve adding constants to the input or to the function itself, a stretch or compression occurs when we multiply the parent function \(f(x)=b^x\) by a constant \(|a|>0\). 

For example, if we begin by graphing the parent function \(f(x)=2^x\), we can then graph the stretch, using \(a=3\), to get \(g(x)=3{(2)}^x\) as shown in Figure (a), and the compression, using \(a=\dfrac{1}{3}\), to get \(h(x)=\dfrac{1}{3}{(2)}^x\) as shown on the right in Figure (b).

VERTICAL STRETCHES AND COMPRESSIONS OF THE PARENT FUNCTION \(y = b^x\)

For any factor \(a \ne 0\), the function \(f(x)=a{(b)}^x\)

• is stretched vertically by a factor of \(a\) if \(|a|>1\).
• is compressed vertically by a factor of \(a\) if \(0 < |a|<1\).
• The new \(y\)-coordinates are equal to \(ay\). This would include vertical reflection if present.
• has a \(y\)-intercept of \((0,a)\).
• has a horizontal asymptote at \(y=0\), a range of \((0,\infty)\), and a domain of \((−\infty,\infty)\), which are unchanged from the parent function.

If a vertically stretched, compressed and/or reflected function also has a vertical shift, like \(g(x)=a{(b)}^x + d, \) then the vertical shift, (\(d\) units up or down), must be done AFTER performing the vertical stretching, compression, and/or reflection.

Example \(\PageIndex{4}\): Graphing the Vertical Stretch of an Exponential Function

Sketch a graph of \(f(x)=4{\Big(\dfrac{1}{2}\Big)}^x\). State the domain, range, and asymptote.

Before graphing, identify the behavior and key points on the graph.

• Since \(b=\dfrac{1}{2}\) is between zero and one, the left tail of the graph will increase without bound as \(x\) decreases, and the right tail will approach the \(x\) -axis as \(x\) increases.
• Since \(a=4\), the graph of the parent function \(y={\Big(\dfrac{1}{2}\Big)}^x\) will be stretched vertically by a factor of \(4\).

• Plot the \(y\) - intercept, \((0,4)\), along with two other points. We can use \((−1,8)\) and \((1,2)\).
• Draw a smooth curve connecting the points, as shown in the figure on the right.

The domain is \((−\infty,\infty)\);  the range is \((0,\infty)\);  the horizontal asymptote is \(y=0\).

Sketch the graph of \(f(x)=\dfrac{1}{2}{(4)}^x\). State the domain, range, and asymptote.

The domain is \((−\infty,\infty)\); the range is \((0,\infty)\); the horizontal asymptote is \(y=0\). 

Combining Transformations of Exponential Functions

Now that we have worked with each type of translation for the exponential function, we can summarize them.

A transformation of an exponential function has the form \(f(x)=ab^{mx+c}+d\) 

The transformations to the parent function, \(y=b^x\), \(b>1\), needed to obtain \(f\) are given below. The order these are done is important. 

• If \(c > 0\) shift left \(c\) units.  
• If \(c < 0\) shift right \(c\) units..  
• If \( m \) is negative, reflect over the \(y\)-axis.   \( \quad \) \( (x \rightarrow -x ).\)
• If \( |m| > 1 \)  shrink the graph horizontally by a factor of \( \frac{1}{|m|}. \)   
• If \( 0<|m|<1 \)  stretch the graph horizontally by a factor of \( \frac{1}{|m|}. \)  
• If \(a\) is negative, reflect over the \(x\) - axis.  \( \quad \) \( ( y \rightarrow -y )  \).
• If \(|a|>0\) stretch the graph vertically by a factor of  \( |a| \).  
• If \(0<|a|<1\)compress the graph vertically by a factor of  \( |a| \).   
• If \(d > 0\) shift up \(d\) units.
• If \(d < 0\) shift down \(d\) units.

If the exponential function is written in the form  \(f(x)=ab^{m(x + c)}+d\) then reverse the order of steps 1 and 2 -- do reflecting and stretching first, then do the horizontal shift.  

Example \(\PageIndex{5}\)

Sketch the graphs of \( g(x) = -2 \cdot 3^{x-5}+6 \) and \( f(x) = 3 \cdot 2^{-x+1}-4 \) and the corresponding original parent function. State the domain, range, and horizontal asymptote of the transformation. 

The transformations needed to obtain the graph of \(g(x)\) from the graph of \(y\) are:

• Shift right 5 units ( \( x  \rightarrow x+5 \) )
• Reflect over the \(x\)-axis  ( \( y  \rightarrow -y \) )
• Vertically stretch by a factor of 2  ( \( y  \rightarrow 2y \) )
• Shift up \(6\) units  ( \( y  \rightarrow y-4 \) )

The graph of \(g(x)\) and its parent function is on the right.

The domain is \((−\infty,\infty)\); the range is \((-\infty, 6)\); the horizontal asymptote is \(y=6\).

If tables are used to graph the function, coordinate points for the parent function appear in the table below

\( \begin{array}{|r|c|c|c|c| c |} \hline \text{Parent function: }x & \text{HA} & -1 & 0 & 1 & 2 \\ \hline  y = 3^x & 0 & \frac{1}{3} & 1 & 3 & 9 \\[2pt] \hline \end{array} \) 

Corresponding coordinate points for the transformation \( g(x) = -2 \cdot 3^{x-5}+6 \) would be

\( \begin{array}{|r|c|c|c|c| c |} \hline \text{New }x  \text{ is: } \:\;\quad x+5 & \text{HA} & 4 & 5 & 6 & 7 \\ \hline \text{New } y  \text{ is: } -2y+6 & 6 & 5\frac{1}{3} & 4 & 0 & -12 \\[2pt] \hline \end{array} \) 

The transformations needed to obtain the graph of \(f(x)\) from the graph of \(y\) are:

• Shift left 1 unit ( \( x  \rightarrow x-1 \) )
• Reflect over the \(y\)-axis  ( \( x  \rightarrow -x \) )
• Vertically stretch by a factor of 3  ( \( y  \rightarrow 3y \) )
• Shift down \(4\) units  ( \( y  \rightarrow y-4 \) )

If instead we re-wrote the function in the form: \( f(x) = 3 \cdot 2^{-(x-1)}-4 \), the transformations would be

• Shift right 1 unit ( \( x  \rightarrow x+1 \) )

The graph of \(f(x)\) and its parent function is on the right.

The domain is \((−\infty,\infty)\); the range is \((\infty, -4)\); the horizontal asymptote is \(y=-4\).

\( \begin{array}{|r|c|c|c|c| c |} \hline \text{Parent function: }x & \text{HA} & -1 & 0 & 1 & 2 \\ \hline  y = 2^x & 0 & \frac{1}{2} & 1 & 2 & 4 \\[2pt] \hline \end{array} \) 

Corresponding coordinate points for the transformation \( f(x) = 3 \cdot 2^{-x+1}-4 \) would be

\( \begin{array}{|r|c|c|c|c| c |} \hline \text{New } x  \text{ is: }  -x+1 & \text{HA} & 2 & 1 & 0 & -1 \\ \hline \text{New } y  \text{ is: } 3y-4 & -4 & -2\frac{1}{2} & -1 & 2 & 8 \\[2pt] \hline \end{array} \) 

Example \(\PageIndex{6}\)

Sketch the graph of \( f(x) = 2 ^ {\frac{1}{2} x + 4} - 3 \). State the domain, range, and horizontal asymptote of the transformation. 

The basic parent function is \( y = 2^x\).  

• Shift left \(4\) units ( \( x  \rightarrow x-4 \) )
• Horizontally stretch by a factor of 2  ( \( x  \rightarrow 2x \) )
• Shift down \(3\) units  ( \( y  \rightarrow y-3 \) )

The domain is \((−\infty,\infty)\); the range is \((-3, \infty)\); the horizontal asymptote is \(y=-3\).

Corresponding coordinate points for the transformation would be

\( \begin{array}{|r|c|c|c|c| c |} \hline \text{New }x  \text{ is: }  2(x-4) & \text{HA} & -10 & -8 & -6 & -4 \\ \hline \text{New } y  \text{ is: } y-3 & -3 & -2.5 & -2 & -1 & 1 \\[2pt] \hline \end{array} \) 

Note that if the function were rewritten in the form: \( f(x) = 2 ^ {\frac{1}{2} (x + 8)} - 3 \) then the transformations indicated by this version of the function would be

• Shift left \(8\) units ( \( x  \rightarrow x-8 \) )

Sketch the graph of \( f(x) = 2 ^ {-2 x - 4} + 3 \). State the domain, range, and horizontal asymptote of the transformation.   

The domain is \((−\infty,\infty)\); the range is \((3, \infty)\); the horizontal asymptote is \(y=3\).

As written the transformations done on the parent function \(y = 2^x\) would be

\( \quad \) right 4, reflect over y axis,  \( x \rightarrow \frac{1}{2} x \), up 3.

However, if rewritten as \( f(x) = 2 ^ {-2 (x +2)} + 3 \), the transformations would be

\( \quad \) reflect over y axis,  \( x \rightarrow \frac{1}{2} x \), left 2, up 3.

Construct an Exponential Equation from a Description 

Example \(\PageIndex{8}\): Write a Function from a Description

Write the equation for the function described below. Give the horizontal asymptote, the domain, and the range.

\(f(x)=e^x\) is vertically stretched by a factor of \(2\) , reflected across the y -axis, and then shifted up \(4\) units.

We want to find an equation of the general form \(f(x)=ab^{x+c}+d\). We use the description provided to find \(a\), \(b\), \(c\), and \(d\). 

• We are given the parent function \(f(x)=e^x\), so \(b=e\).
• The function is vertically stretched by a factor of \(2\), so \(a=2\).
• The function is reflected about the \(y\)-axis. We replace \(x\) with \(−x\) to get: \(e^{−x}\).
• The graph is shifted vertically \(4\) units, so \(d=4\).

Substituting in the general form we get,

\(f(x)=ab^{x+c}+d\)

\(=2e^{−x+0}+4\)

\(=2e^{−x}+4\)

The domain is \((−\infty,\infty)\); the range is \((4,\infty)\); the horizontal asymptote is \(y=4\).

Write the equation for function described below. Give the horizontal asymptote, the domain, and the range.

\(f(x)=e^x\) is compressed vertically by a factor of \(\dfrac{1}{3}\), reflected across the \(x\)-axis and then shifted down \(2\) units.

\(f(x)=−\dfrac{1}{3}e^{x}−2\); the domain is \((−\infty,\infty)\); the range is \((−\infty,2)\); the horizontal asymptote is \(y=2\).

Exponent Properties

When learning about exponents, we are typically given expressions that have a whole number exponent and a base that is a variable, like \(x^2\). These type of expressions are technically called power functions. In contrast, the base can be a constant and the exponent can have a variable in it, like \(2^x\). Expressions in this form are called exponential functions. When simplifying expressions with the variable in the exponent, we often use the Laws of Exponents "backwards".  A review of properties of exponents from a different perspective is briefly discussed here. Competency with exponents is a very useful skill to have. The table below summarizes these properties.

Example \(\PageIndex{9}\)

Rewrite the following exponential expressions such that the result uses only one exponent, \(x\). Simplify.

\( 1.\quad 3 \left( \dfrac{3}{16} \right) ^x \qquad 2. \quad\dfrac{16}{7} \left( \dfrac{8}{49} \right) ^x   \)

Use exponent properties to describe the transformations needed to graph the function \(f(x) = 5 \cdot 3^{-4x+2}-6\) from a parent function.

Key Concepts

• The graph of the function \(f(x)=b^x\) has a y- intercept at \((0, 1)\),domain \((−\infty, \infty)\),range \((0, \infty)\), and horizontal asymptote \(y=0\).
• If \(b>1\),the function is increasing. The left tail of the graph will approach the asymptote \(y=0\), and the right tail will increase without bound.
• If \(0<b<1\), the function is decreasing. The left tail of the graph will increase without bound, and the right tail will approach the asymptote \(y=0\).
• The equation \(f(x)=b^x+d\) represents a vertical shift of the parent function \(f(x)=b^x\).
• The equation \(f(x)=b^{x+c}\) represents a horizontal shift of the parent function \(f(x)=b^x\).
• The equation \(f(x)=ab^x\), where \(a>0\), represents a vertical stretch if \(|a|>1\)  or compression if \(0<|a|<1\) of the parent function \(f(x)=b^x\). 
• When the parent function \(f(x)=b^x\) is multiplied by \(−1\), the result, \(f(x)=−b^x\), is a reflection about the x -axis. When the input is multiplied by \(−1\), the result, \(f(x)=b^{−x}\), is a reflection about the y -axis.
• All translations of the exponential function can be summarized by the general equation \(f(x)=ab^{mx+c}+d\). 
• Using the general equation \(f(x)=ab^{x+c}+d\), we can write the equation of a function given its description. 

Exponential Functions: Graphing

This packet helps students understand how to graph exponential graphs and what exponential graphs look like. Each page starts with easier problems that get more difficult as students work through the packet. After doing all 16 problems, students should be more comfortable doing these problems and have a clear understanding of how to solve them. 

Graph the function using a table of values. Define the domain and range.   

Simple:  y = 2  x  

 Advanced:  y=-(2 x ) 

Practice problems require knowledge of exponents.

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Lesson 7.1 Graphing Exponential Functions

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This includes the student notes, teacher notes, and homework assignment with answer key for the lesson 7.1 Graphing Exponential Functions.

This is part of my Unit 7 Exponential and Log Functions for Algebra 2 .

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