# 4.1 Exponential functions  (Page 2/16)

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Apparently, the difference between “the same percentage” and “the same amount” is quite significant. For exponential growth, over equal increments, the constant multiplicative rate of change resulted in doubling the output whenever the input increased by one. For linear growth, the constant additive rate of change over equal increments resulted in adding 2 to the output whenever the input was increased by one.

The general form of the exponential function is $\text{\hspace{0.17em}}f\left(x\right)=a{b}^{x},\text{\hspace{0.17em}}$ where $\text{\hspace{0.17em}}a\text{\hspace{0.17em}}$ is any nonzero number, $\text{\hspace{0.17em}}b\text{\hspace{0.17em}}$ is a positive real number not equal to 1.

• If $\text{\hspace{0.17em}}b>1,$ the function grows at a rate proportional to its size.
• If $\text{\hspace{0.17em}}0 the function decays at a rate proportional to its size.

Let’s look at the function $\text{\hspace{0.17em}}f\left(x\right)={2}^{x}\text{\hspace{0.17em}}$ from our example. We will create a table ( [link] ) to determine the corresponding outputs over an interval in the domain from $\text{\hspace{0.17em}}-3\text{\hspace{0.17em}}$ to $\text{\hspace{0.17em}}3.$

 $x$ $-3$ $-2$ $-1$ $0$ $1$ $2$ $3$ $f\left(x\right)={2}^{x}$ ${2}^{-3}=\frac{1}{8}$ ${2}^{-2}=\frac{1}{4}$ ${2}^{-1}=\frac{1}{2}$ ${2}^{0}=1$ ${2}^{1}=2$ ${2}^{2}=4$ ${2}^{3}=8$

Let us examine the graph of $\text{\hspace{0.17em}}f\text{\hspace{0.17em}}$ by plotting the ordered pairs we observe on the table in [link] , and then make a few observations.

Let’s define the behavior of the graph of the exponential function $\text{\hspace{0.17em}}f\left(x\right)={2}^{x}\text{\hspace{0.17em}}$ and highlight some its key characteristics.

• the domain is $\text{\hspace{0.17em}}\left(-\infty ,\infty \right),$
• the range is $\text{\hspace{0.17em}}\left(0,\infty \right),$
• as $\text{\hspace{0.17em}}x\to \infty ,f\left(x\right)\to \infty ,$
• as $\text{\hspace{0.17em}}x\to -\infty ,f\left(x\right)\to 0,$
• $\text{\hspace{0.17em}}f\left(x\right)\text{\hspace{0.17em}}$ is always increasing,
• the graph of $\text{\hspace{0.17em}}f\left(x\right)\text{\hspace{0.17em}}$ will never touch the x -axis because base two raised to any exponent never has the result of zero.
• $\text{\hspace{0.17em}}y=0\text{\hspace{0.17em}}$ is the horizontal asymptote.
• the y -intercept is 1.

## Exponential function

For any real number $\text{\hspace{0.17em}}x,$ an exponential function is a function with the form

$f\left(x\right)=a{b}^{x}$

where

• $\text{\hspace{0.17em}}a\text{\hspace{0.17em}}$ is the a non-zero real number called the initial value and
• $\text{\hspace{0.17em}}b\text{\hspace{0.17em}}$ is any positive real number such that $\text{\hspace{0.17em}}b\ne 1.$
• The domain of $\text{\hspace{0.17em}}f\text{\hspace{0.17em}}$ is all real numbers.
• The range of $\text{\hspace{0.17em}}f\text{\hspace{0.17em}}$ is all positive real numbers if $\text{\hspace{0.17em}}a>0.$
• The range of $\text{\hspace{0.17em}}f\text{\hspace{0.17em}}$ is all negative real numbers if $\text{\hspace{0.17em}}a<0.$
• The y -intercept is $\text{\hspace{0.17em}}\left(0,a\right),$ and the horizontal asymptote is $\text{\hspace{0.17em}}y=0.$

## Identifying exponential functions

Which of the following equations are not exponential functions?

• $f\left(x\right)={4}^{3\left(x-2\right)}$
• $g\left(x\right)={x}^{3}$
• $h\left(x\right)={\left(\frac{1}{3}\right)}^{x}$
• $j\left(x\right)={\left(-2\right)}^{x}$

By definition, an exponential function has a constant as a base and an independent variable as an exponent. Thus, $\text{\hspace{0.17em}}g\left(x\right)={x}^{3}\text{\hspace{0.17em}}$ does not represent an exponential function because the base is an independent variable. In fact, $\text{\hspace{0.17em}}g\left(x\right)={x}^{3}\text{\hspace{0.17em}}$ is a power function.

Recall that the base b of an exponential function is always a positive constant, and $\text{\hspace{0.17em}}b\ne 1.\text{\hspace{0.17em}}$ Thus, $\text{\hspace{0.17em}}j\left(x\right)={\left(-2\right)}^{x}\text{\hspace{0.17em}}$ does not represent an exponential function because the base, $\text{\hspace{0.17em}}-2,$ is less than $\text{\hspace{0.17em}}0.$

Which of the following equations represent exponential functions?

• $f\left(x\right)=2{x}^{2}-3x+1$
• $g\left(x\right)={0.875}^{x}$
• $h\left(x\right)=1.75x+2$
• $j\left(x\right)={1095.6}^{-2x}$

$g\left(x\right)={0.875}^{x}\text{\hspace{0.17em}}$ and $j\left(x\right)={1095.6}^{-2x}\text{\hspace{0.17em}}$ represent exponential functions.

## Evaluating exponential functions

Recall that the base of an exponential function must be a positive real number other than $\text{\hspace{0.17em}}1.$ Why do we limit the base $b\text{\hspace{0.17em}}$ to positive values? To ensure that the outputs will be real numbers. Observe what happens if the base is not positive:

• Let $\text{\hspace{0.17em}}b=-9\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}x=\frac{1}{2}.\text{\hspace{0.17em}}$ Then $\text{\hspace{0.17em}}f\left(x\right)=f\left(\frac{1}{2}\right)={\left(-9\right)}^{\frac{1}{2}}=\sqrt{-9},$ which is not a real number.

Why do we limit the base to positive values other than $1?$ Because base $1\text{\hspace{0.17em}}$ results in the constant function. Observe what happens if the base is $1:$

I've run into this: x = r*cos(angle1 + angle2) Which expands to: x = r(cos(angle1)*cos(angle2) - sin(angle1)*sin(angle2)) The r value confuses me here, because distributing it makes: (r*cos(angle2))(cos(angle1) - (r*sin(angle2))(sin(angle1)) How does this make sense? Why does the r distribute once
How can you tell what type of parent function a graph is ?
generally by how the graph looks and understanding what the base parent functions look like and perform on a graph
William
if you have a graphed line, you can have an idea by how the directions of the line turns, i.e. negative, positive, zero
William
y=x will obviously be a straight line with a zero slope
William
y=x^2 will have a parabolic line opening to positive infinity on both sides of the y axis vice versa with y=-x^2 you'll have both ends of the parabolic line pointing downward heading to negative infinity on both sides of the y axis
William
y=x will be a straight line, but it will have a slope of one. Remember, if y=1 then x=1, so for every unit you rise you move over positively one unit. To get a straight line with a slope of 0, set y=1 or any integer.
Aaron
yes, correction on my end, I meant slope of 1 instead of slope of 0
William
what is f(x)=
I don't understand
Joe
Typically a function 'f' will take 'x' as input, and produce 'y' as output. As 'f(x)=y'. According to Google, "The range of a function is the complete set of all possible resulting values of the dependent variable (y, usually), after we have substituted the domain."
Thomas
Sorry, I don't know where the "Â"s came from. They shouldn't be there. Just ignore them. :-)
Thomas
Darius
Thanks.
Thomas
Â
Thomas
It is the Â that should not be there. It doesn't seem to show if encloses in quotation marks. "Â" or 'Â' ... Â
Thomas
Now it shows, go figure?
Thomas
what is this?
i do not understand anything
unknown
lol...it gets better
Darius
I've been struggling so much through all of this. my final is in four weeks 😭
Tiffany
this book is an excellent resource! have you guys ever looked at the online tutoring? there's one that is called "That Tutor Guy" and he goes over a lot of the concepts
Darius
thank you I have heard of him. I should check him out.
Tiffany
is there any question in particular?
Joe
I have always struggled with math. I get lost really easy, if you have any advice for that, it would help tremendously.
Tiffany
Sure, are you in high school or college?
Darius
Hi, apologies for the delayed response. I'm in college.
Tiffany
how to solve polynomial using a calculator
So a horizontal compression by factor of 1/2 is the same as a horizontal stretch by a factor of 2, right?
The center is at (3,4) a focus is at (3,-1), and the lenght of the major axis is 26
The center is at (3,4) a focus is at (3,-1) and the lenght of the major axis is 26 what will be the answer?
Rima
I done know
Joe
What kind of answer is that😑?
Rima
I had just woken up when i got this message
Joe
Rima
i have a question.
Abdul
how do you find the real and complex roots of a polynomial?
Abdul
@abdul with delta maybe which is b(square)-4ac=result then the 1st root -b-radical delta over 2a and the 2nd root -b+radical delta over 2a. I am not sure if this was your question but check it up
Nare
This is the actual question: Find all roots(real and complex) of the polynomial f(x)=6x^3 + x^2 - 4x + 1
Abdul
@Nare please let me know if you can solve it.
Abdul
I have a question
juweeriya
hello guys I'm new here? will you happy with me
mustapha
The average annual population increase of a pack of wolves is 25.
how do you find the period of a sine graph
Period =2π if there is a coefficient (b), just divide the coefficient by 2π to get the new period
Am
if not then how would I find it from a graph
Imani
by looking at the graph, find the distance between two consecutive maximum points (the highest points of the wave). so if the top of one wave is at point A (1,2) and the next top of the wave is at point B (6,2), then the period is 5, the difference of the x-coordinates.
Am
you could also do it with two consecutive minimum points or x-intercepts
Am
I will try that thank u
Imani
Case of Equilateral Hyperbola
ok
Zander
ok
Shella
f(x)=4x+2, find f(3)
Benetta
f(3)=4(3)+2 f(3)=14
lamoussa
14
Vedant
pre calc teacher: "Plug in Plug in...smell's good" f(x)=14
Devante
8x=40
Chris
Explain why log a x is not defined for a < 0
the sum of any two linear polynomial is what
Momo
how can are find the domain and range of a relations
the range is twice of the natural number which is the domain
Morolake
A cell phone company offers two plans for minutes. Plan A: $15 per month and$2 for every 300 texts. Plan B: $25 per month and$0.50 for every 100 texts. How many texts would you need to send per month for plan B to save you money?
6000
Robert
more than 6000
Robert
For Plan A to reach $27/month to surpass Plan B's$26.50 monthly payment, you'll need 3,000 texts which will cost an additional \$10.00. So, for the amount of texts you need to send would need to range between 1-100 texts for the 100th increment, times that by 3 for the additional amount of texts...
Gilbert
...for one text payment for 300 for Plan A. So, that means Plan A; in my opinion is for people with text messaging abilities that their fingers burn the monitor for the cell phone. While Plan B would be for loners that doesn't need their fingers to due the talking; but those texts mean more then...
Gilbert