Direct Comparison Test inequality help

In summary, the Direct Comparison Test allows us to use the inequality Ln n < n^(1/10) even though it is not true for a great many n values because it is only necessary for the statement to hold for values of n that are larger than a certain number, in this case n=3. By looking at the graphs of Ln (n) and n^(k), we can see that the statement is true for a larger interval than just n < 3. Therefore, the Direct Comparison Test is still a valid method for determining convergence even though the inequality is not universally true.
  • #1
dami
18
0

Homework Statement



Explain why the Direct Comparison Test allows us to use the inequality Ln n < n^(1/10) even though it is not true for a great many n values.

Homework Equations





The Attempt at a Solution


I looked at the graphs of Ln (n) vs. n^(k)
 
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  • #2


dami said:

Homework Statement



Explain why the Direct Comparison Test allows us to use the inequality Ln n < n^(1/10) even though it is not true for a great many n values.

Homework Equations





The Attempt at a Solution


I looked at the graphs of Ln (n) vs. n^(k)
And what did you find out?

Also, how did you graph nk?
 
  • #3


Actually, ln n < n^{1/10} is only true for n < 3 (for integer values of n).
 
  • #4


dami said:
Actually, ln n < n^{1/10} is only true for n < 3 (for integer values of n).
That is incorrect. The statement ln n < n.1 is true for n = 3, and it is also true for a lot of much larger values.

In this problem you're supposed to provide justification for the assertion that ln n < n.1 for some infinitely long interval.
 
  • #5


Thanks. Just realized I have been looking at the question the wrong way
 
  • #6


Just plotted the graph
 

1. What is the Direct Comparison Test inequality and how does it work?

The Direct Comparison Test inequality is a method used to determine whether an infinite series converges or diverges. It compares the given series to a known convergent or divergent series, and if the known series is larger than the given series, then the given series must also converge. This test is only applicable for series with positive terms.

2. When should I use the Direct Comparison Test inequality?

The Direct Comparison Test inequality is most useful when the terms of the given series are not easily compared to a known series, such as a geometric or p-series. It can also be used when the Limit Comparison Test is inconclusive.

3. What are the steps for using the Direct Comparison Test inequality?

The steps for using the Direct Comparison Test inequality are as follows:

  1. Identify the given series and the known series to compare it to.
  2. Show that the given series has positive terms.
  3. Show that the known series is larger than the given series.
  4. If the known series converges, then the given series must also converge.
  5. If the known series diverges, then the given series must also diverge.

4. Can the Direct Comparison Test inequality be used for series with negative terms?

No, the Direct Comparison Test inequality can only be used for series with positive terms. If the given series has negative terms, the Absolute Comparison Test must be used instead.

5. Are there any limitations to the Direct Comparison Test inequality?

Yes, the Direct Comparison Test inequality can only be used for series with positive terms and is only applicable when the known series is larger than the given series. It also cannot be used for alternating series or series with terms that do not approach zero.

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