### All GRE Subject Test: Math Resources

## Example Questions

### Example Question #9 : Concepts Of Convergence And Divergence

There are 2 series, and , and they are both convergent. Is convergent, divergent, or inconclusive?

**Possible Answers:**

Inconclusive

Convergent

Divergent

**Correct answer:**

Convergent

Infinite series can be added and subtracted with each other.

Since the 2 series are convergent, the sum of the convergent infinite series is also convergent.

Note: The starting value, in this case n=1, must be the same before adding infinite series together.

### Example Question #10 : Concepts Of Convergence And Divergence

You have a divergent series , and you multiply it by a constant 10. Is the new series convergent or divergent?

**Possible Answers:**

Divergent

Convergent

Inconclusive

**Correct answer:**

Divergent

This is a fundamental property of series.

For any constant c, if is convergent then is convergent, and if is divergent, is divergent.

is divergent in the question, and the constant c is 10 in this case, so is also divergent.

### Example Question #1 : Sequences & Series

There are 2 series and .

Is the sum of these 2 infinite series convergent, divergent, or inconclusive?

**Possible Answers:**

Inconclusive

Divergent

Convergent

**Correct answer:**

Convergent

A way to find out if the sum of the 2 infinite series is convergent or not is to find out whether the individual infinite series are convergent or not.

Test the first series

.

This is a geometric series with .

By the geometric test, this series is convergent.

Test the second series

.

This is a geometric series with .

By the geometric test, this series is convergent.

Since both of the series are convergent, is also convergent.

### Example Question #2 : Sequences & Series

Find the radius of convergence for the power series

**Possible Answers:**

**Correct answer:**

We can use the limit

to find the radius of convergence. We have

This means the radius of convergence is .

### Example Question #3 : Sequences & Series

Determine if the following series is divergent, convergent or neither.

**Possible Answers:**

Divergent

Neither

Convergent

Both

Inconclusive

**Correct answer:**

Divergent

In order to figure if

is convergent, divergent or neither, we need to use the ratio test.

Remember that the ratio test is as follows.

Suppose we have a series . We define,

Then if

, the series is absolutely convergent.

, the series is divergent.

, the series may be divergent, conditionally convergent, or absolutely convergent.

Now lets apply the ratio test to our problem.

Let

and

Now

.

Now lets simplify this expression to

.

Since ,

we have sufficient evidence to conclude that the series is divergent.

### Example Question #1 : Sequences & Series

Calculate the sum of the following infinite geometric series:

**Possible Answers:**

**Correct answer:**

This is an infinite geometric series.

The sum of an infinite geometric series can be calculated with the following formula,

, where is the first value of the summation, and r is the common ratio.

Solution:

Value of can be found by setting

r is the value contained in the exponent

### Example Question #5 : Sequences & Series

Determine how many terms need to be added to approximate the following series within :

**Possible Answers:**

**Correct answer:**

This is an alternating series test.

In order to find the terms necessary to approximate the series within first see if the series is convergent using the alternating series test. If the series converges, find n such that

Step 1:

An alternating series can be identified because terms in the series will “alternate” between + and –, because of

Note: Alternating Series Test can only show convergence. It cannot show divergence.

If the following 2 tests are true, the alternating series converges.

- {} is a decreasing sequence, or in other words

Solution:

1.

2. {} is a decreasing functon, since a factorial never decreases.

Since the 2 tests pass, this series is convergent.

Step 2:

Plug in n values until

4 needs to be added to approximate the sum within .

### Example Question #6 : Sequences & Series

Evaluate: . (Round to 4 places)

**Possible Answers:**

**Correct answer:**

Step 1: Plug in values into the function and add up the fraction:

Step 2: Find the sum of the fractions....

We can convert the fractions to decimals:

Step 3: Round to places...

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