Limit of Series for Infinity Sum: Basic Comparison Test

In summary, to determine if a series for infinity sum n=1 (2n+1)/(5n+1) converges or diverges, it is suitable to use the basic comparison test. By dividing the fraction and examining the terms, we can see that the limit of the associated sequence is not 0, indicating divergence. The use of other tests, such as the n-th term test for divergence, is not necessary but can be an easy check.
  • #1
teng125
416
0
for infinity sum n=1 (2n+1) / (5n+1)

how to find the converges or diverges??
is it suitable to use basic comparison test??pls show me
 
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  • #2
If we split the fraction, we get

[tex]\frac{{2n + 1}}{{5n + 1}} = \frac{2}{5} + \frac{3}{{25n + 5}}[/tex]

As you can see, the second term will go to 0 if n tends to infinity but there's a remaining term 2/5. In other words: the limit of the associated sequence isn't 0 (for n going to infinity) and this is a necessary (though not sufficient) condition for the convergence of the series.
 
  • #3
Or: divide both numerator and denominator by n:
[tex]\frac{2n+1}{5n+1}= \frac{2+\frac{1}{n}}{5+\frac{1}{n}}[/tex]

Now what does 1/n go to as n goes to infinity?
 
  • #4
but the question ask for diverges or converges??and the answer is diverges
 
  • #5
When a series doesn't converge, it diverges.
 
  • #6
is it necessary to use any rules such as the one i mentioned??or just to perform the steps showed above??
 
  • #7
I don't think those are necessary, if the limit of the (positive) sequence doesn't go to 0 (which is easy to check without using those other tests), then the (positive) series doesn't converge.
 
  • #8
teng the n-th term test for divergence is an easy check for this one.
 

What is the Limit of Series for Infinity Sum?

The limit of a series for an infinity sum is a mathematical concept that represents the value that a series approaches as the number of terms in the series increases towards infinity. It is denoted by the symbol ∞ and is used to understand the behavior of a series as it grows.

How is the Limit of Series for Infinity Sum calculated?

The limit of a series for an infinity sum can be calculated using various mathematical techniques, such as the basic comparison test, the ratio test, and the integral test. These methods involve evaluating the series for different values of n and determining if the series converges or diverges as n approaches infinity.

What is the Basic Comparison Test for the Limit of Series for Infinity Sum?

The basic comparison test is a mathematical tool used to determine the convergence or divergence of a series. It involves comparing the given series with a known series that is either convergent or divergent. If the given series is found to be smaller than the known series, then it is also convergent. If it is larger, then it is divergent.

When should the Basic Comparison Test be used?

The basic comparison test should be used when the given series cannot be evaluated using other methods, such as the ratio test or the integral test. It is also useful when the series involves simple terms and can be easily compared to a known series. However, it should be noted that the basic comparison test does not always provide a definitive answer and further analysis may be required.

What are the limitations of the Basic Comparison Test?

While the basic comparison test is a useful tool, it does have its limitations. It can only be used to determine the convergence or divergence of a series, but it does not provide information about the actual value of the limit. It also assumes that the known series used for comparison is known to be convergent or divergent, which may not always be the case.

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