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## Algebra 1

### Unit 12: Lesson 1

Exponential vs. linear growth# Intro to exponential functions

CCSS.Math: , ,

An exponential function represents the relationship between an input and output, where we use repeated multiplication on an initial value to get the output for any given input. Exponential functions can grow or decay very quickly. Exponential functions are often used to model things in the real world, such as populations, radioactive materials, and compound interest. Created by Sal Khan and CK-12 Foundation.

## Want to join the conversation?

- What does he mean by "exploding"(20 votes)
- I guess it's just his way of saying that the graph increases really fast...(2 votes)

- how do you convert graph to equation in exponential functions?(1 vote)
- So the standard form for a quadratic is y=a(b)^x. So one basic parent function is y=2^x (a=1 and b=2). Learning the behavior of the parent functions help determine the how to read the graphs of related functions. You start with no shifts in x or y, so the parent funtion y=2^x has a asymptote at y=0, it goes through the points (0,1) (1,2)(2,4)(3,8),... So we find the common ratio by dividing adjacent terms 8/4=4/2=2/1=2. Thus, we find the base b by dividing the y value of any point by the y value of the point that is 1 less in the x direction which shows an exponential growth. It works the same for decay with points (-3,8). (-2,4)(-1,2)(0,1), So 1/2=2/4=4/8=1/2. It will also have a asymptote at y=0. Next, if we have to deal with a scale factor a, the y intercept will tell us that. With 2(2)^x, you double all the y values to (0,2)(1,4)(2,8)(3,16) - note that 16/8=8/4=4/2=2, so we still get the same base, but the y intercept tells us that the scale factor is 2. Similarly, if we have (0,3) and (1,6) our base is 6/3=2, but the scale factor is 3, so we have y=3(2)^x. This will work the same for decay functions, but the base will be a fraction less than 1.

So the next easiest is to shift up and down by adding a constant to the end. This can be easily be determined by a change in the asymptote. If you see an asymptote at say y=3, then "act like" this is the y axis and see how far points are away from the this line. Thus y=2^x + 3 would have points (0,4) 1 away from asymptote, (1,5) two away from asymptote, etc. Thus, you would have to do (5- 3)/(4 - 3) to get 2/1=2 as the base. This is a good introduction, which is good for all but shifts in the x direction such as y = 3 (4)^(x+2) - 5. This shifts from the origin to (-2,-5) which makes the asymptote at y=-5, but it is a little harder to determine the x axis shift back 2.(20 votes)

- What is the difference between exponential functions and quadratic functions(8 votes)
- Linear functions only have x to the first power

Quadratics have x^2 - shape is a parabola

Exponentials have x in the exponent such as 2^x, 3^x, (1/2)^x - has asymptotes where function tries to reach a y value, but never does(2 votes)

- At1:37, why did you not solve for 3^-1, and skip right to 3^0? Is there a reason or is that just a mistake?(7 votes)
- Sal just made a mistake when he originally wrote the x values in the table, he skipped -1, so when he calculated y values, he skipped 3^-1.(1 vote)

- How do you multiply 1.04 times an exponent of 1/12(4 votes)
- Since there is no rational number multiplied 12 times to get 1.04, you could either leave it that way or use a calculator and put in 1.04^(1/12) and round the answer.(1 vote)

- Why is a function with irrational exponents defined only for a base greater or equal than zero?

Check this out: if you go to Desmos and type in -2^pi you get -8.824

BUT if you type in the equivalent form e^(ln(-2^pi) you get undefined as a result. Can someone shed some light on the reason why this happens?(3 votes) - What if it is 3 raised to power 1.02?(2 votes)
- You would use a calculator to find that value. Here's the approx. value that my calculator created:

3^(1.02) = 3.0666462398354310624879497287303(3 votes)

- why does the base in an exponential function have to be positive?(2 votes)
- If it is negative, the function is
**totally**different. for example:

y = (-2)^x

x = 1: y = -2

x = 2: y = +4

x = 3: y = -8

x = 4: y = +16

etc.

Basically (for x integers), when x is even, y is positive, and when x is odd, y is negative, though the**absolute value**of y**is**growing exponentially.(2 votes)

- How did he get the 1/81 in the problem? Why isn't it negative 81?(2 votes)
- A negative exponent tells you to use the reciprocal of the base to get a positive exponent.

3^(-4) = 1/3^4 = 1/81

The base is not negative. You are multiplying 4 positive numbers together.

1/3^4 = 1/(3*3*3*3) = 1/81

Hope this helps.

You may want to review the lessons on negative exponents to better understand this lesson.(2 votes)

- Whats the exponential form of 10,000(2 votes)
- The exponential form of 10,000 is 10^4 because it is 10 · 10 · 10 · 10. I hope this helps.(2 votes)

## Video transcript

In this video, I want to
introduce you to the idea of an exponential function and
really just show you how fast these things can grow. So let's just write an example
exponential function here. So let's say we have y is equal
to 3 to the x power. Notice, this isn't x to
the third power, this is 3 to the x power. Our independent variable x
is the actual exponent. So let's make a table here to
see how quickly this thing grows, and maybe we'll
graph it as well. So let's take some
x values here. Let's start with x is
equal to negative 4. Then we'll go to negative 3,
negative 2, 0, 1, 2, 3, and 4. And let's figure out what our
y-values are going to be for each of these x-values. Now, here, y is going to be 3 to
the negative 4 power, which is equal to 1 over 3 to
the fourth power. 3 to the third is 27 times
3 again is 81. So this is equal to 1/81. When x is equal to negative
3, y is 3. We'll do this in a
different color. This color is hard to read. y is 3 to the negative
3 power. Well, that's 1 over 3
to the third power, which is equal to 1/27. So we're going from a
super-small number to a less super-small number. And then 3 to the negative
2 power is going to be 1/9, right? 1 over 3 squared, and then we
have 3 to the 0 power, which is just equal to 1. So we're getting a little bit
larger, a little bit larger, but you'll see that we
are about to explode. Now, we have 3 to
the first power. That's equal to 3. So we have 3 to the second
power, right? y is equal to 3 to the second power. That's 9. 3 to the third power, 27. 3 to the fourth power, 81. If we were to put the
fifth power, 243. Let's graph this, just
to get an idea of how quickly we're exploding. Let me draw my axes here. So that's my x-axis and
that is my y-axis. And let me just do it in
increments of 5, because I really want to get the general
shape of the graph here. So let me just draw as straight
a line as I can. Let's say this is 5, 10, 15. Actually, I won't get
to 81 that way. I want to get to 81. Well, that's good enough. Let me draw it a little
bit differently than I've drawn it. So let me draw it down here
because all of these values, you might notice, are positive
values because I have a positive base. So let me draw it like this. Good enough. And then let's say I have 10,
20, 30, 40, 50, 60, 70, 80. That is 80 right there. That's 10. That's 30. That'll be good for
approximation. And then let's say that
this is negative 5. This is positive 5 right here. And actually, let me stretch
it out a little bit more. Let's say this is negative 1,
negative 2, negative 3, negative 4. Then we have 1, 2, 3, and 4. So when x is equal to 0, we're
equal to 1, right? When x is equal to 0, y is equal
to 1, which is right around there. When x is equal to 1, y is equal
to 3, which is right around there. When x is equal to 2, y is equal
to 9, which is right around there. When x equal to 3, y is
equal to 27, which is right around there. When x is equal to 4,
y is equal to 81. You see very quickly this
is just exploding. If I did 5, we'd go to
243, which wouldn't even fit on my screen. When you go to negative 1, we
get smaller and smaller. So at negative 1,
we're at 1/9. Eventually, you're not even
going to see this. It's going to get closer
and closer to zero. As this approaches larger and
larger negative numbers, or I guess I should say smaller
negative numbers, so 3 to the negative thousand, 3 to the
negative million, we're getting numbers closer and
closer to zero without actually ever approaching
zero. So as we go from negative
infinity, x is equal to negative infinity, we're getting
very close to zero, we're slowly getting our
way ourselves away from zero, but then bam! Once we start getting
into the positive numbers, we just explode. We just explode, and we
keep exploding at an ever-increasing rate. So the idea here is just to
show you that exponential functions are really,
really dramatic. Well, you can always construct
a faster expanding function. For example, you could say y is
equal to x to the x, even faster expanding, but out of the
ones that we deal with in everyday life, this is one of
the fastest. So given that, let's do some word problems
that just give us an appreciation for exponential
functions. So let's say that someone
sends out a chain letter in week 1. In week 1, someone sent a chain
letter to 10 people. And the chain letter says you
have to now send this chain letter to 10 more new people,
and if you don't, you're going to have bad luck, and your hair
is going to fall out, and you'll marry a frog,
or whatever else. So all of these people agree and
they go and each send it to 10 people the next week. So in week 2, they go
out and each send it to 10 more people. So each of those original 10
people are each sending out 10 more of the letters. So now 100 people have
the letters, right? Each of those 10 sent
out 10, so that 100 letters were sent out. 10 were sent. Here, 100 were sent. In week 3, what's
going to happen? Each of those 100 people who
got this one, they're each going to send out 10, assuming
that everyone is really into chain letters. So 1,000 people are
going to get it. And so the general pattern
here is, the people who receive it, so in week n where
n is the week we're talking about, how many people
received the letter? In week n, we have 10 to the nth
people receive-- i before e except after c-- the letter. So if I were to ask you, how
many people are getting the letter on the sixth week? How many people are actually
going to receive that letter? Well, what's 10 to
the sixth power? 10 to the sixth is equal to 1
with six zeroes, which is 1 million people are going to
receive that letter in just 6 weeks, just sending out
10 letters each. And obviously, in the real
world, most people chuck these in the basket, so you don't have
this good of a hit rate. But if you did, if every 10
people you sent it to also sent it to 10 people and so on
and so forth, by the sixth week, you would have
a million people. And by the ninth week, you would
have a billion people. And frankly, the week
after that, you would run out of people. I'll see you in the
next video.