Sum of a geometric series up to infinity

In summary, a geometric series is a sequence of numbers where each term is obtained by multiplying the previous term by a fixed number. The formula for finding the sum of a geometric series up to infinity is S = a / (1-r). A geometric series will converge if the absolute value of the common ratio "r" is less than 1 and will diverge if the absolute value of "r" is greater than or equal to 1. A geometric series can have a negative common ratio, and the sum can be found up to a certain number of terms using the formula Sn = a (1-r^n) / (1-r).
  • #1
Alekz
1
0

Homework Statement


A geometric series had first term 54 and 4th term 2.
(i) What is the common ratio?
(ii) Find the sum to infinity of the series.
(iii) After how many terms is the sum of the series greater than 99% of the sum to infinity?


Homework Equations


N/A


The Attempt at a Solution


(i) Is obviously 1/3 (54,18,6,2...)
I've no idea how to attempt ii or iii, if someone could explain this I'd be grateful
 
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  • #2
Welcome to PF!

Hi Alekz! Welcome to PF! :smile:

Start by writing the nth term as a function of n :wink:
 

1. What is a geometric series?

A geometric series is a sequence of numbers where each term is obtained by multiplying the previous term by a fixed number. The first term is usually denoted as "a" and the fixed number is denoted as "r". The general form of a geometric series is a, ar, ar^2, ar^3, ..., ar^n.

2. What is the formula for finding the sum of a geometric series up to infinity?

The formula for finding the sum of a geometric series up to infinity is S = a / (1-r), where "a" is the first term and "r" is the common ratio.

3. How do you know when a geometric series converges or diverges?

A geometric series will converge (have a finite sum) if the absolute value of the common ratio "r" is less than 1. If the absolute value of "r" is greater than or equal to 1, the series will diverge (have an infinite sum).

4. Can a geometric series have a negative common ratio?

Yes, a geometric series can have a negative common ratio. This means that the series will alternate between positive and negative terms, but the overall sum may still converge or diverge depending on the value of "r".

5. How do you find the sum of a geometric series up to a certain number of terms?

The formula for finding the sum of a geometric series up to a certain number of terms (n) is Sn = a (1-r^n) / (1-r), where "a" is the first term and "r" is the common ratio. This formula can be used to find the partial sum of a geometric series up to any number of terms.

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