GRE Subject Test: Math : Sequences & Series

Study concepts, example questions & explanations for GRE Subject Test: Math

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Example Questions

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Example Question #26 : Series In Calculus

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

Possible Answers:

Divergent

Inconclusive

Convergent

Correct answer:

Convergent

Explanation:

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 #27 : Series In Calculus

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

Possible Answers:

Convergent

Divergent

Inconclusive

Correct answer:

Divergent

Explanation:

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 #161 : Gre Subject Test: Math

There are 2 series and .

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

Possible Answers:

Divergent

Convergent

Inconclusive

Correct answer:

Convergent

Explanation:

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 #1 : Sequences & Series

Find the radius of convergence for the power series

Possible Answers:

Correct answer:

Explanation:

We can use the limit

to find the radius of convergence. We have

This means the radius of convergence is .

Example Question #67 : Convergence And Divergence

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

 

Possible Answers:

Divergent

Neither

Both

Inconclusive

Convergent

Correct answer:

Divergent

Explanation:

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:

Explanation:

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 #62 : Types Of Series

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

Possible Answers:

Correct answer:

Explanation:

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.

  1.        
  2.        {} 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 #1 : Sequences & Series

Evaluate: . (Round to 4 places)

Possible Answers:

Correct answer:

Explanation:

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...

Example Question #1 : Sequences & Series

Which of the following are not infinite sequences?

Possible Answers:

Correct answer:

Explanation:

Step 1: Define what an infinite sequence is...

An infinite sequence is a sequence that is non-terminating.

Step 2: Determine if each sequence above is infinite...

For , the sequence is always infinite because the set of factorials is infinite. Also, the set of values by raising two factorial powers together is also infinite, it never has an ending term.

For , this sequence is FINITE! 

For , this sequence is FINITE!

 

Example Question #1 : Finding Terms Of A Sequence

If a sequence is 9, 3, -3, -9, -15, ...

What is the 10th term of the sequence?

Possible Answers:

Correct answer:

Explanation:

It will be helpful if you can see that the sequence is changing by -6 to get to each number. The quickest way to solve this is to keep going with the sequence by subtracting six from each number to get to the next number.

9, 3, -3, -9, -15, -21, -27, -33, -39, -45

-45 is the 10th number in the sequence.

Another way to find the 10th term is by using the equation for arithmetic sequences.

where  is the term in question,  is the first term,  is the numbered term, and  is the difference of the terms.

To find d subtract the first two terms.

Therefore, the equation becomes

 

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