What is the sum to infinity of geometric progressions?

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Discussion Overview

The discussion revolves around the concept of the sum to infinity of geometric progressions, specifically examining the series 4, 2, 1, ... with a common ratio of 1/2. Participants explore the nature of this sum, questioning whether it is exactly 8 or merely approaches 8.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant expresses uncertainty about whether the sum to infinity is exactly 8 or just very close to it, indicating a conceptual struggle with the idea of limits.
  • Another participant provides the formula for the sum of a geometric progression and confirms that the sum to infinity is 8 when applying the formula to the given series.
  • A different participant asserts that the sum is definitively 8, emphasizing that it is the limit of finite subsums and that any lesser value would contradict properties of the real number system.
  • One participant notes that when summing a finite number of terms, such as up to n=10,000,000, the result will be slightly less than 8, but the infinite sum is exactly 8.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the interpretation of the sum to infinity, with some asserting it is exactly 8 and others questioning this conclusion. The discussion remains unresolved as differing perspectives on the nature of limits and sums are presented.

Contextual Notes

There are limitations in the discussion regarding the understanding of limits and the definitions involved in summing infinite series, which may affect participants' interpretations.

antevante
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Hi!
In a geometric progression you can find the sum to infinety is some series, for example 4,2,1,... where the common factor is 1/2. The sum to infinety will then be, 8, it says in my book, but I can only think of it as very, very close to 8, not eight exactly. How is it? Is the sum to infinety 8 or just very close to eight?
/Andreas
 
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Sum of a GP = a(1-r^n)/(1-r) where a is the first number in the sequence, r is the common ratio and n is the term number.

When -1<r<1 and n approaches infinity r^n approaches 0. Therefore Sum to infinity = a/(1-r)

Therefore Sum to infinity = 4/(1-0.5) = 8
 
The sum *is* eight. It is the limit of the finite subsums. If it weren't eight but were less than 8, then yo'ud have a problem since one of the (increasing) finite subsums would be greater than your preferred infinite sum. It is a property of the real number system that the sum is 8. It is, by definition, 8 there is no contention about that, if you think it is something different then you don't understand what the words mean.
 
If you were to stop somewhere short of "infinity", say summing up to n= 10000000, then the answer, one of the "subsums" that matt grime referred to (I would say "partial sum") would be slightly less than 8. Summing all terms, that is, never stopping, will give exactly 8.
 

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