# Calculus 2 : Power Series

## Example Questions

### Example Question #72 : Taylor And Maclaurin Series

Find the radius of convergence for the power series .

Explanation:

You might already recognize this as the power series representation for . Since  is well defined for all , the radius of convergence is .

If we want to find the radius of covergence using convergence tests, we can use the Ratio Test here. We have-

Since this limit equals  regardless of the value of , and the Ratio Test indicates absolute convergence of a series when the above limit is less than  converges for all . Hence the radius of convergence is .

### Example Question #1 : Power Series

One useful way of determining convergence for power series is via Asymptotic Comparison.

Which of the following functions grows fastest?

Explanation:

The question does not ask which function is the largest at any given point, it asks which grows fastest. For this question we need to look at all of the terms and determine which function has the dominant term. In this case  is the dominate term therefore, it will grow the fastest.

### Example Question #2 : Power Series

Express  as a power series.

Explanation:

Write the correct definition of cosine as a power series.

Replace  with the term .

### Example Question #3 : Power Series

What is the radius of convergence for the following power series?

Explanation:

With every power series, you must first start by using the ratio test.

The  series is .

Using the ratio test you get

.

Canceling things out and taking the  out of the limit (as you are only taking the limit as n approaches infinity) gets you the following:

, which becomes x (as the limit is 1).

Because the ratio test states that the series can only converge if the absolute value of x is less than or equal to 1, this means that x is between -1 and 1. And so the radius of convergence is 1.

### Example Question #4 : Power Series

Determine the Taylor Series of   at .

Explanation:

Recall that a Taylor Series approximation at  is given by

.

In our case,

So, our final summation will look like

.

### Example Question #5 : Power Series

Find the Maclaurin Series for the function .

Explanation:

We can use the common Maclaurin Series expansion,

and modify it to fit our current function .

First, we substitute  in for . Doing so we obtain,

.

The remaining task is to get the in front of the function. We do this by simply multiplying each side of the above equation by .

Doing so, we obtain,

.

Thus our Maclaurin Series for is,

.

Note: One could also try to use the Maclaurin Series formula  to compute the series directly. However, computing for this function for every proves to be a formidable task. Therefore, it is easiest to solve these problems by remembering the Maclaurin Series for specific functions, and then using substitutions and multiplication tricks to build up to the function in question. I suggest memorizing the following Maclaurin Series':

There are a few others that are useful, but these are the most commonly used in classroom and exam settings.

### Example Question #3107 : Calculus Ii

Find a convergent power series representation for the function. Base the derivation of the power series on a convergent geometric series.

Explanation:

To find a convergent power series representation of the function we can use the following theorem for an infinite geometric series that converges to a finite value:

_________________________________________________________

for  such that

________________________________________________________

First factor the function to so that one of the factors is comparable to the form

Now we can use the theorem to write the  factor as a convergent geometric series. Here our  "" is just . Also,

The series we will derive a power series that will converge to the factor . Also note that we do not have to "show" that  to apply the theorem. We simply write it as an infinite series and then state that  is a constraint on where the power series representation is valid. In other words, it's not a hypothesis we have to verify or check for.

Using the theorem for a convergent infinite series, valid in this case for , we can write the factor as follows:

Now simply bring back the other factor, , and the pull it into the summation.

To find where the power series we've obtained converges, we simply have to apply the condition for the convergence of the geometric series . Even though the geometric series is not the actual power series we obtained for the function, it is still the series we based the power series representation on. All we did was multiply the function  to each term in the summation. Therefore the interval of convergence- which is just the range over  for which the original function converges to the power series representation- must satisfy

This is equivalent to the interval  . Note that the interval is open at  and .

### Example Question #6 : Power Series

Find a series representation of

.

Explanation:

This is an indefinite integral so we can eliminate answers without the +C.

First, we need to find a power series representation for sin(x), which is

.

We then substitute in  to get

and then integrate the general term .

becomes  to get the correct answer, remembering our +C.