# Precalculus : Arithmetic and Geometric Series

## Example Questions

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### Example Question #1 : Sequences And Series

Evaluate:

None of the other answers are correct.

Explanation:

This sum can be determined using the formula for the sum of an infinite geometric series, with initial term  and common ratio :

### Example Question #2 : Sequences And Series

The fourth term in an arithmetic sequence is -20, and the eighth term is -10. What is the hundredth term in the sequence?

105

110

220

55

210

220

Explanation:

An arithmetic sequence is one in which there is a common difference between consecutive terms. For example, the sequence {2, 5, 8, 11} is an arithmetic sequence, because each term can be found by adding three to the term before it.

Let  denote the nth term of the sequence. Then the following formula can be used for arithmetic sequences in general:

, where d is the common difference between two consecutive terms.

We are given the 4th and 8th terms in the sequence, so we can write the following equations:

.

We now have a system of two equations with two unknowns:

Let us solve this system by subtracting the equation  from the equation . The result of this subtraction is

.

This means that d = 2.5.

Using the equation , we can find the first term of the sequence.

Ultimately, we are asked to find the hundredth term of the sequence.

The answer is 220.

### Example Question #1 : Sequences And Series

What is the lowest value of  where the sum of the arithmetic sequence  where  will exceed 200?

Explanation:

The sum of all odd numbers is another way to construct perfect squares.  To see why this is, we can construct the series as follows.

We draw  from the series by subtracting one from each term.

We discard the 0 term and factor 2 out of the remaining terms.

And finally we use the property that  to evaluate the series.

The smallest value of  where the square exceeds 200 is .

### Example Question #2 : Sequences And Series

The first term in an arithmetic series is 3, and the 9th term is 35.  What is the 17th term?

Explanation:

The terms of an arithmetic series are generated by the relation

,

where is the 1st term, is the nth term, and d is the common difference.

For

,

for

.

The first step is to find .

, so

.

Now to find , when .

Use the generating relation

.

### Example Question #1 : Sequences And Series

Find the next term in the series: , , , .

Explanation:

To find the next term, we need to figure out what is happening from one term to the next.

From 2 to 5, we can see that 3 is added.

From 5 to 14, 9 is added.

From 14 to 41, 27 is added.

If you look closely, you can notice a trend.

The amount added each time triples. Therefore, the next amount added should be

Thus,

### Example Question #4 : Sequences And Series

What type of series is listed below?

P-series

None of the answers available

Fibonacci

Geometric

Arithmetic

Arithmetic

Explanation:

In the series given,  is added to each previous term to get the next term. Since a fixed number is ADDED each time, this series can be categorized as an arithmetic series.

### Example Question #2 : Sequences And Series

What type of series is indicated below?

Geometric

P-series

Arithmetic

Constant

None of the given choices

Geometric

Explanation:

First, we need to figure out what the pattern is in this series. Notice how each term results in the following term. In this case, each term is multiplied by  to get the next term. Since each term is MULTIPLIED by a fixed number, this can be defined as a geometric series.

### Example Question #3 : Sequences And Series

What is the common ratio of the Geometric series pictured below?

Explanation:

Common ratio is the number that is multiplied by each term to get the next term in a geometric series. Since the first two terms are  and , we look at what is multiplied between these. Once way to determine this if not immediately obvious is to divide the second term by the first term. In this case we get:

which gives us our common ratio.

### Example Question #1 : Arithmetic And Geometric Series

Find the value of the sum:

Explanation:

This equation is a series in summation notation.

We can see that the bottom k=3 designates where the series starts, 8 represents the stop point, and 1/k represents the rule for summation. We can expand this equation as follows:

Here, we have just substituted "k" for each value from 3 to 8. To solve, we must then find the least common demoninator. That would be 280. This can be found in several ways, such as separating the fractions by like denominators:

### Example Question #8 : Sequences And Series

What type of sequence is the following?

Geometric

Both

Arithmetic

Neither

Geometric

Explanation:

We note that there is no common difference between  and  so the sequence cannot be arithmetic.

We also note that there exists a common ratio between two consecutive terms.

Since there exists a common ratio, the sequence is Geometric.

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