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Intro to geometric sequences

Sal introduces geometric sequences and their main features, the initial term and the common ratio. Created by Sal Khan.

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  • leafers tree style avatar for user Beorn MacRupert
    Can anyone explain to me why the geometric sequence follows the basic structure of an exponential function [f(x) = a * r^x] and why the arithmetic sequence follows the structure of a linear function [f(x) = mx + b]? I get that they are similar concepts but this intrigues me and I would like to understand the reasons behind this similarity. Thanks in advance.
    (35 votes)
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    • blobby green style avatar for user InnocentRealist
      The sequences are in fact linear and exponential functions, but with the domain restricted to positive integers.

      The "standard form" of arithmetic sequences, "a(i) = a(1) + d(i-1)", as a function in the xy plane would be "f(x) = m(x-1) + b", where b = a(1) and m = d; which is a shift to the right by one unit of "f(x) = mx + b". This is because the common difference is not added to the first term in this sequence, while in "y = mx + b", m is added to f(x) at x = 1. When the sequence is given as "a(j) = a(1) + dj" (i.e. the common difference is added to the first term) it translates directly to "y = mx + b" with y = 0*x + b when x=0).
      Similarly, the 1st term of a geometric sequence is in general independent of the common ratio. So the formula should be "a(i) = a(1)*r(i-1)" (shifted to the right), whereas as a function of real numbers an exponential is "y = (initial value)*r^x"

      (Good question).
      (24 votes)
  • blobby green style avatar for user Arbaaz Ibrahim
    Wouldn't the first table be incorrect ( in terms of the jump).
    On the second jump, you would do 0.6 times 120 + 120 because 120 is the jump and, 06 times 120 is after the bounce?
    (4 votes)
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    • piceratops ultimate style avatar for user Wei Wuxian
      Geometric sequences differ from arithmetic sequences. In geometric sequences, to get from one term to another, you multiply, not add. So if the first term is 120, and the "distance" (number to multiply other number by) is 0.6, the second term would be 72, the third would be 43.2, and so on.
      (14 votes)
  • duskpin ultimate style avatar for user Débora Romagnolo
    every sequence usually begins with "a1", why we can't begin with "a0" ? I studied the definition of the sequences as functions, that says a sequence is a function with the positive integer as your domain, and the real numbers as your counter domain. Why we can't use the 0 in this domain too?
    (2 votes)
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    • purple pi purple style avatar for user doctorfoxphd
      You will find that sequences don't always start with a(1). Back here where math is simpler, we do often talk about a(1) as the first item.
      It is really a semantics thing. If you mean that the a(1) is your first term, then you cannot have a zero term. Our terms are discrete.
      However, you can define your first term as a(0) in the same way that in a computer array, the first element is the 0th item. It is usually easier for humans to keep track if the first item is called the n = 1 item.
      When we use sequences to match or model actual occurrences, it can get pretty interesting. In the case that Sal is modeling, the first thing that happens is slightly different, so we call it "0"

      Whichever way you start numbering, it is always important to check that your formula for the sequence actually ends up with the sequence that you want. If you start at a(1), you will usually need to have your exponent as the expression n-1 to match the sequence that you are given.
      On the other hand, if your sequence starts with a(0), you will often find your exponent needs to be n in order for your initial value to be correct for building the sequence you want.
      (15 votes)
  • blobby green style avatar for user Arbaaz Ibrahim
    Why did we start at zero bounces instead of 1. In arithmetic we start at 1?
    (1 vote)
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    • cacteye blue style avatar for user Jerry Nilsson
      Yeah, I would say Sal overthought it a bit...

      Obviously, there's only one jump and the first bounce has to occur after that jump.

      – – –

      So, after her initial (and only) jump Anne reaches a point where the bungee rope will cease extending and start retracting, which I would say counts as her 1st bounce.
      At this point the bungee rope measures a length of 120 ft.

      Then she is pulled upwards for a couple of seconds until gravity again gets the upper hand and pulls her down to her 2nd bounce, at which point the length of the rope is 60% of the length it measured at the previous bounce, i.e. 120 × 0.6 = 72 ft.

      Then at her 3rd bounce, the length of the rope would be
      120 × 0.6² = 43.2 ft.

      Following the same logic, the length of the rope at Anne's 12th bounce would be 120 × 0.6¹¹ ≈ 0.435 ft.
      (15 votes)
  • leaf green style avatar for user Ken Otis
    Sal, I'm more confused than when I started. This video is convoluted.
    (7 votes)
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  • winston baby style avatar for user AJ
    Why is it called "Geometric"? Isn't Geometry about shapes (e.g., triangle, rectangle, etc)?
    (4 votes)
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  • male robot donald style avatar for user Amaka Amah Ugochukwu
    when Sal introduced "n", why was it introduced? and does the letter stand for something? I got lost once that "n" was introduced.
    (4 votes)
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    • duskpin sapling style avatar for user Iris Nogueroles Langa
      n is the placement number or position of the term you want to find. The first term is always n=1, the second term is n=2, the third term is n=3 and so on. Therefore, this is not the value of the term itself but instead the place it has in the geometric sequence.

      Saying "the nth term" means you can calculate the value in position n, allowing you to find any number in the sequence. You use n in the general formula of a geometric sequence and replace it with a number when you want to find the term in a certain position.

      For example, if you have the general formula Un = 100 x (2)^n-1, you can use this to find any number in the sequence.

      U1 is the first term and U1 = 100 x (2)^1-1 = 100 x (2)^0 = 100 x 1 = 100
      U14 is the fourteenth term and U14 = 100 x (2)^14-1 = 100 x (2)^13 = 819200

      Hope this helps!
      (4 votes)
  • aqualine ultimate style avatar for user 24macdonaldlb
    @ We see our first geometric sequence. I know with arithmetic sequences we're always adding, are we always multiplying with geometric sequences?
    (3 votes)
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  • aqualine seedling style avatar for user Sabeen
    At sal multiplies 90 by -1/3. Why is it only mulitplication in geometric sequences but not divison?
    (3 votes)
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  • duskpin sapling style avatar for user dairyqueengirl
    I always remind myself that no matter how hard it is, I Khan always do it.
    (4 votes)
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Video transcript

In this video I want to introduce you to the idea of a geometric sequence. And I have a ton of more advanced videos on the topic, but it's really a good place to start, just to understand what we're talking about when someone tells you a geometric sequence. Now a good starting point is just, what is a sequence? And a sequence is, you can imagine, just a progression of numbers. So for example, and this isn't even a geometric series, if I just said 1, 2, 3, 4, 5. This is a sequence of numbers. It's not a geometric sequence, but it is a sequence. A geometric sequence is a special progression, or a special sequence, of numbers, where each successive number is a fixed multiple of the number before it. Let me explain what I'm saying. So let's say my first number is 2 and then I multiply 2 by the number 3. So I multiply it by 3, I get 6. And then I multiply 6 times the number 3, and I get 18. Then I multiply 18 times the number 3, and I get 54. And I just keep going that way. So I just keep multiplying by the number 3. So I started, if we want to get some notation here, this is my first term. We'll call it a1 for my sequence. And each time I'm multiplying it by a common number, and that number is often called the common ratio. So in this case, a1 is equal to 2, and my common ratio is equal to 3. So if someone were to tell you, hey, you've got a geometric sequence. a1 is equal to 90 and your common ratio is equal to negative 1/3. That means that the first term of your sequence is 90. The second term is negative 1/3 times 90. Which is what? That's negative 30, right? 1/3 times 90 is 30, and then you put the negative number. Then the next number is going to be 1/3 times this. So negative 1/3 times this. 1/3 times 30 is 10. The negatives cancel out, so you get positive 10. Then the next number is going to be 10 times negative 1/3, or negative 10/3. And then the next number is going to be negative 10/3 times negative 1/3 so it's going to be positive 10/3. And you could just keep going on with this sequence. So that's what people talk about when they mean a geometric sequence. I want to make one little distinction here. This always used to confuse me because the terms are used very often in the same context. These are sequences. These are kind of a progression of numbers. 2, then 6, then 18, 90, then negative 30, then 10, then negative 10/3. Then, I'm sorry, this is positive 10/9, right? Negative 1/3 times negative 10/3, negatives cancel out. Right. 10/9. Don't want to make a mistake here. These are sequences. You might also see the word a series. And you might even see a geometric series. A series, the most conventional use of the word series, means a sum of a sequence. So for example, this is a geometric sequence. A geometric series would be 90 plus negative 30, plus 10, plus negative 10/3, plus 10/9. So a general way to view it is that a series is the sum of a sequence. I just want to make that clear because that used to confuse me a lot when I first learned about these things. But anyway, let's go back to the notion of a geometric sequence, and actually do a word problem that deals with one of these. So they're telling us that Anne goes bungee jumping off of a bridge above water. On the initial jump, the cord stretches by 120 feet. So on a1, our initial jump, the cord stretches by 120 feet. We could write it this way. We could write, jump, and then how much the cord stretches. So on the initial jump, on jump one, the cord stretches 120 feet. Then it says, on the next bounce, the stretch is 60% of the original jump, and then each additional bounce stretches the rope 60% of the previous stretch. So here, the common ratio, where each successive term in our sequence is going to be 60% of the previous term. Or it's going to be 0.6 times the previous term. So on the second jump, we're going to start 60% of that, or 0.6 times 120. Which is equal to what? That's equal to 72. Then on the third jump, we're going to stretch 0.6 of 72, or 0.6 times this. So it would be 0.6 times 0.6 times 120. Notice, over here, so on the fourth jump we're going to have 0.6 times 0.6 times 0.6 times 120. 60% of this jump, so every time we're 60% of the previous jump. So if we wanted to make a general formula for this, just based on the way we've defined it right here. So the stretch on the nth jump, what would it be? So let's see, we start at 120 times 0.6 to the what? To the n minus 1. How did I get this? Let me write this a little bit here. So this is equal to 0.6, actually let me write the 120 first. This is equal to 120 times 0.6 to the n minus 1. How did I get that? Well we're defining the first jump as stretching 120 feet. So when you put n is equal to 1 here, you get 1 minus 1, 0. So you have 0.6 to the 0th power, and you've just got a 1 here. And that's exactly what happened on the first jump. Then on the second jump, you put a 2 minus 1, and notice 2 minus 1 is the first power, and we have exactly one 0.6 here. So I figured it was n minus 1 because when n is 2, we have one 0.6, when n is 3, we have two 0.6's multiplied by themselves. When n is 4, we have 0.6 to the third power. So whatever n is, we're taking 0.6 to the n minus 1 power, and of course we're multiplying that times 120. Now and the question they also ask us, what will be the rope stretch on the 12th bounce? And over here I'm going to use the calculator. and actually let me correct this a little bit. This isn't incorrect, but they're talking about the bounce, and we could call the jump the zeroth bounce. Let me change that. This isn't wrong, but I think this is where they're going with the problem. So you can view the initial stretch as the zeroth bounce. So instead of labeling it jump, let me label it bounce. So the initial stretch is the zeroth bounce, then this would be the first bounce, the second bounce, the third bounce. And then our formula becomes a lot simpler. Because if you said the stretch on nth bounce, then the formula just becomes 0.6 to the n times 120, right? On the zeroth bounce, that was our original stretch, you get 0.6 to the 0, that's 1 times 120. On the first bounce, 0.6 to the 1, one 0.6 right here. 0.6 times the previous stretch, or the previous bounce. So this has it in terms of bounces, which I think is what the questioner wants us to do. So what about the 12th bounce? Using this convention right there. So if we do the 12th bounce, let's just get our calculator out. We're going to have 120 times 0.6 to the 12th power. And hopefully we'll get order of operations right, because exponents take precedence over multiplication, so it'll just take the 0.6 to the 12th power only. And so this is equal to 0.26 feet. So after your 12th bounce, she's going to be barely moving. She's going to be moving about 3 inches on that 12th bounce. Well, hopefully you found this helpful. And I apologize for the slight divergence here, but I actually think on some level that's instructive. Because you always have to make sure that your n matches well with what your results are. So when I talked about your first jump, I said, OK this is 1. And then I had 0.6 to the zeroth power, so I did n minus 1. But then when I relabeled things in terms of bounces, this was the zeroth bounce. This makes sense that this is 0.6 to the zeroth power. This is the first bounce, so this would be 0.6 to the first power. Second bounce, 0.6 to the second power. It made our equation a little bit simpler. Anyway, hopefully you found that Interesting.