Help with Resistance Homework: Finding Sum to Infinity

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SUMMARY

The discussion centers on solving a homework problem involving a convergent geometric progression (g.p.) with a first term of 3. The key equations derived are S(5) = 3(r^5 - 1)/(r - 1) and S(10) = 3(r^10 - 1)/(r - 1), leading to the equation 8^10 - 9r^5 + 1 = 0. The correct sum to infinity of the progression is determined to be 8.8, despite an initial incorrect assertion of 1.60. The critical insight is that the sum of the first five terms equals twice the sum of the first ten terms, which is essential for solving for the common ratio r.

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Homework Statement


The first term of a convergent g.p is 3. The sum of the first five terms of the progression is twice the sum of the first ten terms. Find the sum to infinity of the progression.


Homework Equations



S(5)= 3(r^5 -1)/4
S(10)=3(r^10 -1)/9
.
.
.
8^10-9r^5+1=0
Sum to infinity= 8.8

The Attempt at a Solution


But the answer is 1.60
 
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How did you get

[tex]S_5=\frac{3\left(r^5-1\right)}{4}[/tex]

It should be,

[tex]S_5=\frac{3\left(r^5-1\right)}{r-1}[/tex]

Firstly set up your summations correctly, and use the info that is given - mainly:

The sum of the first five terms of the progression is twice the sum of the first ten terms.

This should give you an equality in r, which can then be solved. Then use the infinite summation formula.
 

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