Radius of Convergence for Σ6n(x-5)n(n+1)/(n+11) Series | Solve for x

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SUMMARY

The discussion focuses on determining the radius of convergence for the series Σ6n(x-5)n(n+1)/(n+11). The ratio test was applied incorrectly, leading to an erroneous conclusion about the convergence interval. The correct application of the ratio test involves evaluating the limit of the ratio of consecutive terms, specifically lim (n→inf) f(n+1)/f(n), which should yield the correct bounds for x. The correct interval for convergence is derived from the inequality -1 < 1/(6(x-5)) < 1.

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ReidMerrill
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Homework Statement


Find all values of x such that the given series would converge

Σ6n(x-5)n(n+1)/(n+11)

Homework Equations

The Attempt at a Solution


By doing the ratio test I found that
lim 6n(x-5)n(n+1)/(n+11) * (n+12)/[6n+1(x-5)n+1(n+2)]
n→inf

equals 1/(6(x-5)) * lim (n+12)(n+1)/(n+11)(n+2)
This limit = 1 so to solve for the x I set
-1<1/6(x-5) and 1/6(x-5)<1 and found the (31/6)<x<(29/6)
but apparently this is incorrect. What am I doing wrong?
 
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ReidMerrill said:

Homework Statement


Find all values of x such that the given series would converge

Σ6n(x-5)n(n+1)/(n+11)

Homework Equations

The Attempt at a Solution


By doing the ratio test I found that
lim 6n(x-5)n(n+1)/(n+11) * (n+12)/[6n+1(x-5)n+1(n+2)]
n→inf
Here it looks like you flipped the wrong fraction.
Ratio test is more like \lim_{n\rightarrow \infty} {\frac{f(n+1)}{f(n)}}
Where f is the function under the sigma. In my understanding, this would flip the fraction the other way from what you have.
 
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