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## Algebra (all content)

### Course: Algebra (all content)>Unit 18

Lesson 4: Finite geometric series

# 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?

• 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?
• 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.
• 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.
• 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.
• Does the k of the sigma notation equal the n of the equation?
• 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!
• At (this is a suggestion), why couldn't you have solved 3^100-1?
• 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.
• I'm confused about what happens to the negative at
• you have (2*(1-3^100))/(-2).
then the 2s cancel out: (1-3^100)/(-1).
negative from the -1 goes to the top: -(1-3^100)/1.
1 in denominator goes away: -(1-3^100).
distribute negative: -1+3^100
rewrite order: 3^100-1
• Why does 2*3 to the 0th power equal to 2? Shouldn't it be zero?
• So n in this context is the number of whole numbers the finite geometric series has ?
(1 vote)
• Yes, I think so. In the case in the video, n = 100
• Could we write `Σ2(3^(k-1)) for k=1 to 100` instead of `Σ2(3^k) for k=0 to 99`? What's the preferred way of writing this?
(1 vote)
• Yes you can, that's called 'reindexing'. Neither is preferred, there may be contexts where you want one form or the other.
• 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)
• 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