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## Algebra I (2018 edition)

### Unit 13: 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.

## 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.