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## Algebra 1

### Course: Algebra 1 > Unit 12

Lesson 6: Exponential functions from tables & graphs- Writing exponential functions
- Writing exponential functions from tables
- Exponential functions from tables & graphs
- Writing exponential functions from graphs
- Analyzing tables of exponential functions
- Analyzing graphs of exponential functions
- Analyzing graphs of exponential functions: negative initial value
- Modeling with basic exponential functions word problem
- Connecting exponential graphs with contexts

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# Analyzing graphs of exponential functions

Given the graph of an exponential function, Sal finds the formula of the function and a value that is outside the graph.

## Want to join the conversation?

- What if I only know 2 distant points, let's say (6,28) and (30,8) , how do I find the equation ?(3 votes)
- Megan's comment is correct if you don't know what type of function it is. If you knew it was an exponential function of the form a*(r)^x, the you could construct a system of equations to get you the equation. For example, the exponential equation that roughly goes through the points (6, 28) and (30, 8) is:

y = 38.1*(0.95)^x

You can graph the function on desmos to see for yourself. Hope this helps!(3 votes)

- btw :)

1/81 = 0.012345679012345678(2 votes)- Actually, 1/81 is a repeating decimal with a 9 digit repeat pattern. 1/81 = 0.012345679012345679012345679... with the digits 012345679 repeating.(7 votes)

- where can I learn about exponentials and economy?(2 votes)
- Just search for those term in Khan Academy, you should be able to find some video to view, some article to read, and some practice to work on.(5 votes)

- Why did you write 1^6/3? Is it right? Shouldn't we write (1/3)^6 instead?(2 votes)

## Video transcript

- [Voiceover] So we have the graph of an exponential function here,
and the function is m of x. And what I want to do
is figure out what is m of six going to be equal to? And like always, pause the video, and see if you can work it out. Well, as I mentioned, this
is an exponential function, so m is going to take the form. Let me write it this way. M of x is going to take the form a times r to the xth power, where a is our initial power,
and r is our common ratio. Well the initial value is
pretty straightforward. It's just going to be what m of zero is. So a is going to be equal to m of zero, and we can just look at this graph. When x is equal to zero, the
function is equal to nine. So it's equal to nine, and now we need to figure out our common ratio. So let me set up a little
bit of a table here, just to help us with this. So let me draw some straight lines. And so this is x and m of x. We already know that when x is zero, m of x is equal to nine. We also know when x is, let's see. When x is one, when x
is one, m of x is three. M of x is three. So when we increase our x by one, what happened to our m of x? Well, what did we have to multiply it by? Well, to go from nine to three, you multiplied by 1/3. So that's going to be our common ratio. And in fact, if we wanted to care what m of two is going to be, we
would multiply by 1/3 again. And m of two should be equal to one, and we see that right over here. M of two is, indeed, equal to one. So our common ratio, our common ratio right over here is equal to 1/3. So m of x, we can write it as, m of x is going to be equal to
our initial value, a, which we already figured it out, as a is, a is equal to nine, so
it's going to be nine, times our common ratio,
times our common ratio, 1/3 to the xth power. So I was able to figure out
the formula for our definition for m of x, but that's not what I wanted. I just wanted to figure out
what m of six is going to be. So we can write down that m of six, m of six is going to be nine times one over three to the sixth power. Let's see, that is going to be equal to, that's the same thing as nine times, well one to the six is just one. That's just going to be one to the six, which is just one, over
three to the sixth power. Now what is three to the sixth power? In fact, I can even simplify
this a little bit more. I can recognize that
nine is three squared, so I could say this is going to be three squared over three to the sixth. Three squared over three to the sixth, and then, I could tackle
this a couple of ways. I could just divide the
numerator into the denominator by three squared, in
which case, I would get one over three to the fourth power. Or another way to think
about it, this should be the same thing as three
to the two minus sixth power, which is the same thing as three
to the negative four power, which is, of course, the same thing as one over three to the fourth. So what's three to the fourth? So three squared is nine. Three to the third is 27. Three to the fourth is 81. So this is going to be
equal to one over 81. M of six is equal to one over 81. We could also, we could've done that if we kept going by our table. M of three, multiply it by 1/3, is going to be 1/3. M of four, multiply by 1/3 again, is going to be 1/9. Then we could, m of five is going to be 1/27, and m of six is going to be 181st.