Proof without words: Geometric series

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Discussion Overview

The discussion revolves around a "proof without words" for the sum of the infinite geometric series, specifically focusing on the geometric constructions presented in a linked document. Participants explore the validity of the proof, the relationships between similar triangles, and the construction of diminishing quadrilaterals.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions the validity of the proof, seeking clarification on the order of constructions and the arguments used.
  • Another participant notes the similarity between triangles PQR and TSP, suggesting that the tangent values are the same.
  • There is a discussion about the construction of a line that represents the geometric sum, with some participants expressing confusion about this aspect.
  • Participants identify that the diminishing quadrilaterals are similar to each other, with each having two sides the same and one side being a factor of r times those two sides.
  • Clarification is provided that each quadrilateral is constructed by being attached to the previous one, resulting in a reduction by a factor of r.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the proof and its constructions. While some points are clarified, there remains uncertainty about the initial construction of the quadrilaterals and the overall validity of the proof.

Contextual Notes

Participants have not fully resolved the assumptions underlying the geometric constructions or the specific steps taken in the proof. There are also unresolved questions about the initial conditions of the proof.

Bipolarity
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On this website http://www.albany.edu/~bd445/Eco_466Y/Slides/Infinite_Geometric_Sum,_Proof_Without_Words.pdf there is a "proof without words" of the sum of the infinite geometric series. However, I don't understand what makes the proof valid. In what order were the constructions done etc.? What were the arguments of the proof?

I am referring to the very first picture with the similar triangles.

BiP
 
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Hi Bipolarity! :smile:

Triangle PQR is similar to triangle TSP.

So the two tans are the same. :wink:
 
tiny-tim said:
Hi Bipolarity! :smile:

Triangle PQR is similar to triangle TSP.

So the two tans are the same. :wink:

I get that part, but I don't understand how you can construct the line that is equal to the geometric sum.

BiP
 
oh i see! :smile:

those diminishing quadrilaterals are similar to each other …

each has two sides the same, and one side r times those two :wink:
 
tiny-tim said:
oh i see! :smile:

those diminishing quadrilaterals are similar to each other …

each has two sides the same, and one side r times those two :wink:

I don't understand how the quadrilaterals were constructed in the first place?

BiP
 
each quadrilateral is stuck onto the previous one, so they're each reduced by r :wink:
 
I get it now. Thanks.

BiP
 

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