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Use the Root Test to determine whether the following series converge. $$\sum_{k=1}^{\infty} \frac{k^{2}}{2^{k}}$$

Short Answer

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Question: Determine whether the series converges or diverges using the Root Test: $$\sum_{k=1}^{\infty} \frac{k^{2}}{2^{k}}$$. Answer: The series converges.

Step by step solution

01

Define the Root Test formula

The Root Test formula is given by: $$\lim_{k\to\infty} \sqrt[k]{|a_k|}$$ Where \(a_k\) is the general term of the given series. In this case, the given series is: $$\sum_{k=1}^{\infty} \frac{k^{2}}{2^{k}}$$ We can rewrite it by specifying the general term \(a_k\) as: $$a_k = \frac{k^2}{2^k}$$
02

Apply the Root Test formula to the series

Plug the general term \(a_k\) into the Root Test formula and compute the limit: $$\lim_{k\to\infty} \sqrt[k]{\left|\frac{k^2}{2^k}\right|}$$
03

Simplify the expression inside the limit

For \(k \ge 1\), the expression inside the limit is always positive, so we can remove the absolute value: $$\lim_{k\to\infty} \sqrt[k]{\frac{k^2}{2^k}}$$
04

Manipulate the fraction inside the limit

Using the properties of exponents, we can rewrite the expression inside the limit as: $$\lim_{k\to\infty} \frac{\sqrt[k]{k^2}}{\sqrt[k]{2^k}}$$
05

Simplify the expression inside the limit further

Using properties of exponents, we can rewrite the expression inside the limit as: $$\lim_{k\to\infty} \frac{k^{\frac{2}{k}}}{2}$$
06

Calculate the limit

As we let \(k \to \infty\), \(k^{\frac{2}{k}}\) approaches 1, so the limit becomes: $$\lim_{k\to\infty} \frac{k^{\frac{2}{k}}}{2} = \frac{1}{2}$$
07

Compare the result

Compare the limit to 1: $$\frac{1}{2} < 1$$
08

Determine if the series converges or diverges

Since the limit is less than 1, the Root Test indicates that the series: $$\sum_{k=1}^{\infty} \frac{k^{2}}{2^{k}}$$ converges.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Series Convergence
Understanding series convergence is essential for analyzing infinite sums and their behavior. In simple terms, a series converges if the sum of its infinite terms approaches a finite value. Conversely, it diverges if the sum does not stabilize and instead heads towards infinity.

There are several methods to test for series convergence, and one such method is the Root Test. The Root Test evaluates the convergence of a series by looking at the behavior of its nth root terms as the index goes to infinity. If the limit of the nth root of the absolute value of the terms is less than one, the series converges absolutely. If it is greater than one, the series diverges.

When applying the Root Test, there can sometimes be confusion when handling the expressions within the limit. It is important to remember that we are interested in the long-term behavior as the index increases without bound. Through practice and repetition, students will become more comfortable with the ins-and-outs of this convergence test and its applications to a range of problems.
Limit Calculation
The pursuit of solving a limit lies at the heart of understanding the behavior of functions as they approach specific points or infinity. The calculation of a limit focuses on understanding what value a function, or an expression, approaches as the variable heads towards a certain value.

In the context of the Root Test, we're interested in evaluating the limit of the nth root of the given series' term. This assessment can sometimes demand the application of exponent rules or l'Hôpital's rule, in cases when the limit is indeterminate. The key aspect here is to maintain precision in manipulating expressions to allow for a clear path towards the limit value.

Students must ensure that each step in the limit calculation is justified, especially when simplifying expressions or performing operations that affect the entire term, like taking roots. Mastery of limit calculations is not only fundamental in series convergence but also throughout calculus and higher mathematics.
Exponent Properties
Exponents are the shorthand for repeated multiplication and come with a set of rules, or properties, that simplify complex expressions. Understanding these rules is crucial for manipulating and simplifying series and limit problems in calculus.

Key properties include the product of powers, quotient of powers, and power of a power rule. For example, the power of a product rule states that \( (a \cdot b)^n = a^n \cdot b^n \). These properties are invaluable tools when dealing with series where each term involves exponentiation.

In the Root Test example, we utilized these properties to manipulate the series' terms into a form that reveals the nature of its convergence. Breaking down such complex terms into simpler parts is a strategy that makes limit calculation significantly more manageable. It's often the clarity in understanding exponent rules that dictates whether a student can seamlessly work through series and calculus problems.

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Most popular questions from this chapter

Repeated square roots Consider the expression \(\sqrt{1+\sqrt{1+\sqrt{1+\sqrt{1+\cdots}}}},\) where the process continues indefinitely. a. Show that this expression can be built in steps using the recurrence relation \(a_{0}=1, a_{n+1}=\sqrt{1+a_{n}}\), for \(n=0,1,2,3, \ldots . .\) Explain why the value of the expression can be interpreted as \(\lim _{n \rightarrow \infty} a_{n},\) provided the limit exists. b. Evaluate the first five terms of the sequence \(\left\\{a_{n}\right\\}\) c. Estimate the limit of the sequence. Compare your estimate with \((1+\sqrt{5}) / 2,\) a number known as the golden mean. d. Assuming the limit exists, use the method of Example 5 to determine the limit exactly. e. Repeat the preceding analysis for the expression \(\sqrt{p+\sqrt{p+\sqrt{p+\sqrt{p+\cdots}}},}\) where \(p>0 .\) Make a table showing the approximate value of this expression for various values of \(p .\) Does the expression seem to have a limit for all positive values of \(p ?\)

James begins a savings plan in which he deposits \(\$ 100\) at the beginning of each month into an account that earns \(9 \%\) interest annually or, equivalently, \(0.75 \%\) per month. To be clear, on the first day of each month, the bank adds \(0.75 \%\) of the current balance as interest, and then James deposits \(\$ 100\). Let \(B_{n}\) be the balance in the account after the \(n\) th deposit, where \(B_{0}=\$ 0\). a. Write the first five terms of the sequence \(\left\\{B_{n}\right\\}\). b. Find a recurrence relation that generates the sequence \(\left\\{B_{n}\right\\}\). c. How many months are needed to reach a balance of \(\$ 5000 ?\)

Consider the following sequences defined by a recurrence relation. Use a calculator, analytical methods, and/or graphing to make a conjecture about the limit of the sequence or state that the sequence diverges. $$a_{n+1}=\frac{1}{2}\left(a_{n}+2 / a_{n}\right) ; a_{0}=2$$

Suppose that you take 200 mg of an antibiotic every 6 hr. The half-life of the drug is 6 hr (the time it takes for half of the drug to be eliminated from your blood). Use infinite series to find the long-term (steady-state) amount of antibiotic in your blood.

Consider the sequence \(\left\\{x_{n}\right\\}\) defined for \(n=1,2,3, \ldots\) by $$x_{n}=\sum_{k=n+1}^{2 n} \frac{1}{k}=\frac{1}{n+1}+\frac{1}{n+2}+\dots+\frac{1}{2 n}.$$ a. Write out the terms \(x_{1}, x_{2}, x_{3}\). b. Show that \(\frac{1}{2} \leq x_{n}<1,\) for \(n=1,2,3, \ldots\). c. Show that \(x_{n}\) is the right Riemann sum for \(\int_{1}^{2} \frac{d x}{x}\) using \(n\) subintervals. d. Conclude that \(\lim _{n \rightarrow \infty} x_{n}=\ln 2\).

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