Science:Math Exam Resources/Courses/MATH105/April 2013/Question 02 (a)

MATH105 April 2013

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Question 02 (a)
Find out whether the numerical series below converges or diverges. You should provide appropriate justification in order to receive credit. (a) $\sum _{k=2}^{\infty }{\frac {\sqrt[{3}]{k}}{k^{2}-k}}\,.$

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hints below. Read the first one and consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! If after a while you are still stuck, go for the next hint.

Hint 1
Try using the limit comparison test. You may also want to remind yourself of the p-series test.

Hint 2
Consider the dominant powers in the numerator and denominator of the terms in the series when determining a series to compare to when using the limit comparison test.

Checking a solution serves two purposes: helping you if, after having used all the hints, you still are stuck on the problem; or if you have solved the problem and would like to check your work. If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work. If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. For this question, we apply the limit comparision test. Let $a_{k}={\frac {\sqrt[{3}]{k}}{k^{2}-k}}.$ We wish to define another series $\sum b_{k}$ such that $c=\lim _{k\rightarrow \infty }{\frac {a_{k}}{b_{k}}}$ is positive and finite (i.e. $<\infty$). By comparing the dominant powers in the numerator (1/3) and denominator (2), we are inspired (1/3-2 = -5/3) to define $b_{k}$ as $\displaystyle b_{k}=k^{-5/3}={\frac {1}{k^{5/3}}}.$ The terms $a_{k}$, $b_{k}$ are positive for all $k\geq 2$. By the limit comparison test, if $c$ is positive and finite, then both series converge or both diverge. We can see that $\displaystyle \sum _{k}b_{k}$ will converge by the p-series test, since the power on $k$ in the denominator, $5/3$, is greater than $1$. So if $c$ is finite, then $\displaystyle \sum _{k}a_{k}$ converges. ${\begin{aligned}c=\lim _{k\rightarrow \infty }{\frac {{\sqrt[{3}]{k}}\times k^{5/3}}{k^{2}-k}}=\lim _{k\rightarrow \infty }{\frac {k^{2}}{k^{2}-k}}=\lim _{k\rightarrow \infty }{\frac {1}{1-1/k}}=1<\infty .\end{aligned}}$ So $c$ is positive and finite. Therefore, by the limit comparison test, the sum ${\color {blue}\sum _{k=2}^{\infty }{\frac {\sqrt[{3}]{k}}{k^{2}-k}}}$ converges.

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