# Precalculus : Sums of Infinite Series

## Example Questions

### Example Question #1 : Sums Of Infinite Series

Find the value for

Explanation:

To best understand, let's write out the series. So

We can see this is an infinite geometric series with each successive term being multiplied by .

A definition you may wish to remember is

where  stands for the common ratio between the numbers, which in this case is  or . So we get

### Example Question #2 : Sums Of Infinite Series

Evaluate:

The series does not converge.

Explanation:

This is a geometric series whose first term is   and whose common ratio is . The sum of this series is:

### Example Question #3 : Sums Of Infinite Series

Evaluate:

The series does not converge.

Explanation:

This is a geometric series whose first term is   and whose common ratio is . The sum of this series is:

### Example Question #4 : Sums Of Infinite Series

What is the sum of the following infinite series?

diverges

Explanation:

This series is not alternating - it is the mixture of two geometric series.

The first series has the positive terms.

The second series has the negative terms.

The sum of these values is 3.5.

### Example Question #5 : Sums Of Infinite Series

What is the sum of the alternating series below?

Explanation:

The alternating series follows a geometric pattern.

We can evaluate the geometric series from the formula.

### Example Question #6 : Sums Of Infinite Series

Find the sum of the following infinite series:

Explanation:

Notice that this is an infinite geometric series, with ratio of terms = 1/3. Hence it can be rewritten as:

Since the ratio, 1/3, has absolute value less than 1, we can find the sum using this formula:

Where  is the first term of the sequence. In this case , and thus:

### Example Question #7 : Sums Of Infinite Series

In the infinite series  each term  such that the first two terms are  and .  What is the sum of the first eight terms in the series?

-64

170

210

128

-256

170

Explanation:

Once you're identified the pattern in the series, you might see a quick way to perform the summation. Since the base of the exponent for each term is negative, the result will be positive if  is even, and negative if it is odd. And the series will just list the first 8 powers of 2, with that positive/negative rule attached. So you have:

-2, 4, -8, 16, -32, 64, -128, 256

Note that each "pair" of adjacent numbers has one negative and one positive. for the first pair, -2 + 4 = 2.  For the second, -8 + 16 = 8.  For the third, -32 + 64 = 32. And so for the fourth, -128 + 256 = 128.  You can then quickly sum the values to see that the answer is 170.