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

In summary, to find all values of x such that the given series would converge, the ratio test was performed and it was found that the limit equals 1/(6(x-5)), which means that x must be between (31/6) and (29/6). However, in the attempt at a solution, it appears that the fraction was flipped incorrectly. The correct ratio test should be lim 6n(x-5)n(n+1)/(n+11) * (n+12)/[6n+1(x-5)n+1(n+2)] as n approaches infinity.
  • #1
ReidMerrill
66
2

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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  • #2
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 [tex]\lim_{n\rightarrow \infty} {\frac{f(n+1)}{f(n)}}[/tex]
Where [itex]f[/itex] 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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Related to Radius of Convergence for Σ6n(x-5)n(n+1)/(n+11) Series | Solve for x

1. What is the radius of convergence?

The radius of convergence is a mathematical concept used in power series to determine the interval in which the series converges. It represents the distance from the center of the series to the nearest point where the series converges.

2. How is the radius of convergence calculated?

The radius of convergence is calculated using the ratio test, which compares the absolute value of the ratio between consecutive terms in the series to a limit. If the limit is less than 1, the series converges; if the limit is greater than 1, the series diverges; and if the limit is equal to 1, the test is inconclusive.

3. What is the significance of the radius of convergence?

The radius of convergence is important because it determines the range of values for which the power series can be used to approximate the function it represents. If a value falls outside of the radius of convergence, the series will not converge and therefore cannot be used to approximate the function at that point.

4. Can the radius of convergence be negative?

No, the radius of convergence must always be a positive value. This is because it represents a distance, and distances cannot be negative. However, the interval of convergence can be negative if the series converges for negative values of x.

5. How does the radius of convergence relate to the convergence of the series?

The radius of convergence is directly related to the convergence of the series. If the series converges at a specific value of x, then that value must fall within the radius of convergence. Similarly, if the series diverges at a specific value of x, then that value must fall outside of the radius of convergence.

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