If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Worked example: finite geometric series (sigma notation)

Sal evaluates the geometric series Σ2(3ᵏ) for k=0 to 99 using the finite geometric series formula a(1-rⁿ)/(1-r).

Want to join the conversation?

  • piceratops seedling style avatar for user teddyhoisington
    My professor handed me a sheet listing the different formulas for geometric series and it shows up as Sn = a*(r^n -1) / r-1
    Will this give a different answer than your formula of Sn = a*(1-r^n) / 1-r?
    (10 votes)
    Default Khan Academy avatar avatar for user
    • female robot grace style avatar for user Emily H
      The two formulas are equivalent. If you multiply Sn = a*(r^n-1) / r-1 by -1 / -1, it will not change the value because -1 / -1 = 1.
      a*(r^n-1) * -1 = -a*(r^n-1) = a*(-r^n+1) = a*(1-r^n).
      r-1 * -1 = -r+1 = 1-r.
      Thus the formula becomes Sn = a*(1-r^n) / 1-r, which means the two formulas are equivalent.
      (23 votes)
  • aqualine tree style avatar for user Charlie  Jyu
    Does the k of the sigma notation equal the n of the equation?
    (7 votes)
    Default Khan Academy avatar avatar for user
    • aqualine tree style avatar for user Judith Gibson
      Good question!
      No, it doesn't, and it's important to understand the difference.
      We're talking about 2 things - the "S sub n" equation and the sigma notation of the series.
      The "n" in the"S sub n" equation only represents how many terms there are all together in the series.
      So, "S sub 100" means the sum of the first 100 terms in the series.
      The k of the sigma notation tells us what needs to be substituted into the expression in the sigma notation in order to get the full series of terms.
      So, if k goes from 0 to 99, there are 100 terms, so 100 would be used as "n" in the "S sub n" equation.
      If k goes from 3 to 24, there are 22 terms, so 22 would be used as "n" in the "S sub n" equation.
      (If desired, the individual terms of the series could be found by substituting each of the "k" values into the sigma notation expression.)
      Hope this helps -- even if only in a small way!
      (22 votes)
  • purple pi purple style avatar for user David Taylor
    There is no mention of the formula for geometric series in the previous videos. It is just introduced here as though it should be something already learned. It is only explained in the "Finite geometric series formula justification" which is the last video in this section.
    (13 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user Behrooz Rahn
      I agree. No there is no mention of it.It is like you say,it is only explained later on the last section of the tutorial. It should be corrected as it is confusing and not typical of the lessons where every step follows the previous one.
      (10 votes)
  • male robot hal style avatar for user johrisuyash1
    At (this is a suggestion), why couldn't you have solved 3^100-1?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • duskpin sapling style avatar for user Vu
      We could but it's difficult to do without a calculator, because 3¹⁰⁰ (that is 3 times itself 100 times) is a very huge number. My calculator shows the result a number that is around 40 digits. Even with calculator, most regular calculators wouldn't be able to display all of its digits.
      (6 votes)
  • old spice man green style avatar for user Paul
    I'm confused about what happens to the negative at
    (3 votes)
    Default Khan Academy avatar avatar for user
  • starky ultimate style avatar for user Andres Romero
    So n in this context is the number of whole numbers the finite geometric series has ?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • eggleston blue style avatar for user dena escot
    what is the difference between when k=0 then No of terms will be 100 and when K=1 then No of terms will be 99 ?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • leaf green style avatar for user cossine
      Hi Dena, I have answer similar question on finite geometric series fomula.

      Question: Is it possible to find n by using a formula, as it is with arithmetic series?

      The video is actually about geometric series, however it is useful some knowledge regarding arithmetic series.

      It will depend on the exact question.

      How many number are there from 0-150?

      Ans: 150 - 0 + 1 = 151

      There is the plus one because we need to include 0.

      How many numbers are there in the given sequence:

      0, 2, 4, ...., 20

      If we divide by 2 we get:

      0, 1, 2, ..., 10:

      Ans: 10 - 0 + 1 = 11 numbers


      How many numbers are there in the sequence:

      7, 9, 11, ..., 21

      Subtract by 7 to get:

      0, 2, 4,..., 14

      Divide by 2:

      0, 1, 2, ..., 7

      Therefore the amount of numbers is 7-0+1 = 8
      (2 votes)
  • leaf green style avatar for user Gerrit Jan Brethouwer
    I don't understand the last(arithmatic part of it.
    How do I get from
    4(1-(2)^50) to -4.5*10^15
    Could you please explain?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • leaf yellow style avatar for user Howard Bradley
      Well it takes a LOT of arithmetic (or a calculator). 2⁵⁰ = 2 * 2 * 2 ... (for fifty 2's) is a BIG number.
      Using a calculator, I get 1.126 x 10¹⁵. It's so large that taking it away from one effectively just negates it. And finally multiplying by 4 gives the result shown.
      (2 votes)
  • blobby green style avatar for user Amaria Gonzales
    At the end why isn't is -3^100 - 1?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • orange juice squid orange style avatar for user OrangeBullet
    Why, in Finite geometric series, does it sometimes not give me a calculator when I am computing things like 4(1-(2)^50)?
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

- [Voiceover] Let's do some examples where we're finding sums of finite geometric series. Now let's just remind ourselves in a previous video we derived the formula where the sum of the first n terms is equal to our first term times one minus our common ratio to the nth power all over one minus our common ratio. So let's apply that to this finite geometric series right over here. So what is our first term and what is our common ratio? And what is our n? Well, some of you might just be able to pick it out by inspecting this here, but for the sake of this example, let's expand this out a little bit. This is going to be equal to two times three to the zero, which is just two, plus two times three to the first power, plus two times three to the second power, I can write first power there, plus two times three to the third power, and we're gonna go all the way to two times three to the 99th power. So what is our first term? What is our a? Well, a is going to be two. And we see that in all of these terms here. So a is going to be two. What is r? Well, each successive term, as k increases by one, we're multiplying by three again. So, three is our common ratio. So that right over there, that is r. Let me make sure that we, that is a. And now what is n going to be? Well, you might be tempted to say, well, we're going up to k equals 99, maybe n is 99, but we have to realize that we're starting at k equals zero. So there is actually 100 terms here. Notice, when k equals zero, that's our first term, when k equals one, that's our second term, when k equals two, that's our third term, when k equals three, that's our fourth term, when k equals 99, this is our 100th term, 100th term. So what we really want to find is S sub 100. So let's write that down, S sub 100, for this geometric series is going to be equal to two times one minus three to the 100th power, to the 100th power, all of that, all of that over, all of that over one minus three. And we could simplify this, I mean at this point it is arithmetic that you'd be dealing with, but down here you would have a negative two, and so you'd have two divided by negative two so that is just a negative. And so negative of one minus three to the 100th, that's the same thing, this is equal to three to the 100th, three to the 100th power minus one. And we're done.