# Calculus 2 : Ratio Test

## Example Questions

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### Example Question #101 : Polynomial Approximations And Series

Which of these series cannot be tested for convergence/divergence properly using the ratio test? (Which of these series fails the ratio test?)

Explanation:

The ratio test fails when . Otherwise the series converges absolutely if , and diverges if .

Testing the series , we have

Hence the ratio test fails here. (It is likely obvious to the reader that this series diverges already. However, we must remember that all intuition in mathematics requires rigorous justification. We are attempting that here.)

### Example Question #102 : Polynomial Approximations And Series

Assuming that , . Using the ratio test, what can we say about the series:

It is convergent.

We cannot conclude when we use the ratio test.

We cannot conclude when we use the ratio test.

Explanation:

As required by this question we will have to use the ratio test.  if L<1 the series converges absolutely, L>1 the series diverges, and if L=1 the series could either converge or diverge.

To do so, we will need to compute : . In our case:

Therefore

.

We know that

This means that

Since L=1 by the ratio test, we can't conclude about the convergence of the series.

### Example Question #1 : Ratio Test

We consider the series : , use the ratio test to determine the type of convergence of the series.

The series is fast convergent.

We cannot conclude about the nature of the series.

It is clearly divergent.

We cannot conclude about the nature of the series.

Explanation:

To be able to use to conclude using the ratio test, we will need to first compute the ratio then use  if L<1 the series converges absolutely, L>1 the series diverges, and if L=1 the series could either converge or diverge. Computing the ratio we get,

.

We have then:

Therefore have :

It is clear that .

By the ratio test , we can't conclude about the nature of the series.

### Example Question #1 : Ratio Test

Using the ratio test,

what can we say about the series.

where  is an integer that satisfies:

We can't conclude when we use the ratio test.

We can't use the ratio test to study this series.

We can't conclude when we use the ratio test.

Explanation:

Let be the general term of the series. We will use the ratio test to check the convergence of the series.

The Ratio Test states:

then if,

1) L<1 the series converges absolutely.

2) L>1 the series diverges.

3) L=1 the series either converges or diverges.

Therefore we need to evaluate,

we have,

therefore:

.

We know that

and therefore,

This means that :

By the ratio test we can't conclude about the nature of the series. We will have to use another test.

### Example Question #4 : Convergence And Divergence

Consider the following series :

where is given by:

. Using the ratio test, find the nature of the series.

We can't conclude when using the ratio test.

The series is convergent.

We can't conclude when using the ratio test.

Explanation:

Let be the general term of the series. We will use the ratio test to check the convergence of the series.

if L<1 the series converges absolutely, L>1 the series diverges, and if L=1 the series could either converge or diverge.

We need to evaluate,

we have:

.

Therefore:

. We know that,

and therefore

This means that :

.

By the ratio test we can't conclude about the nature of the series. We will have to use another test.

### Example Question #2 : Ratio Test

Use the ratio test to determine whether the series below is convergent or divergent.

The series is convegent.

The series is divergent.

The series is divergent.

Explanation:

To use the ratio test, we will need to compute the ratio

. Then  if L<1 the series converges absolutely, L>1 the series diverges, and if L=1 the series either converges or diverges.

We have  we have then :

.

Since we can write :

Thus  and because  the series must diverge.

Therefore we conclude that the series is divergent.

### Example Question #1 : Convergence And Divergence

Using the ratio test , what can you say about the following series:

The series is convergent.

The series has two limits.

The series will converge and diverge when it gets close to .

The series is divergent.

The series is convergent.

Explanation:

We will use the comparison test to conclude about the convergence of this series. To show that the majorant series is convergent we will have to call upon the ratio test.

if L<1 the series converges absolutely, L>1 the series diverges, and if L=1 the series either converges or diverges.

We note first that,

where n is positive integer.

We have . By the comparison test, if we can show that the series is convergent, then by the comparison test, the series is also convergent.

We consider now the series :. We have:

and since   we conclude that the series is convergent by the ratio test.

This shows that our series is convergent.

### Example Question #4 : Ratio Test

Suppose that a series has positive terms.

If  what can we say about the convergency of the series.

We will need to know the first two terms.

We will need to know the explicit formula for .

The series is convergent.

The series is divergent.

We can't conclude.

The series is divergent.

Explanation:

We are given that the series has positive terms. We know that

. This means that .

Now we note that .

Then  if L<1 the series converges absolutely, L>1 the series diverges, and if L=1 the series either converges or diverges.

Since  the series will diverge.

The ratio test lets us conclude that the series is divergent.

### Example Question #5 : Ratio Test

We will consider the following series :

.

What can you say about the nature of this series using the ratio test? Assume that .

We need to know the exact value of .

The series converges to .

The series is convergent.

We can't conclude about the nature of the series.

The nature of the series depends on .

We can't conclude about the nature of the series.

Explanation:

Note that for and the series is always positive.

To be able to use the ratio test, we will have to compute the ratio:

. Then find . If L<1 the series converges absolutely, L>1 the series diverges, and if L=1 the series either converges or diverges.

We have hence :

Therefore :

since

By the ratio test we cannot conclude about the nature of the series.

### Example Question #6 : Ratio Test

We consider the following series:

where .

Using the ratio test what can you say about the nature of the series?

Is it convergent or divergent?

The series is divergent.

We can't conclude using the ratio test.

The series is convergent.

We can't conclude using the ratio test.

Explanation:

We will use the ratio test noting first that the series is positive.

We will compute the ratio:

. Note that:

Hence :

Now we have and

and we have

if L<1 the series converges absolutely, L>1 the series diverges, and if L=1 the series either converges or diverges.

Therefore the ratio test is inconclusive. We will need to use another test .

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