College Algebra 2e

# 9.4Series and Their Notations

College Algebra 2e9.4 Series and Their Notations

### Learning Objectives

In this section, you will:

• Use summation notation.
• Use the formula for the sum of the ﬁrst n terms of an arithmetic series.
• Use the formula for the sum of the ﬁrst n terms of a geometric series.
• Use the formula for the sum of an inﬁnite geometric series.
• Solve annuity problems.

### Try It #8

At a new job, an employee’s starting salary is $32,100. She receives a 2% annual raise. How much will she have earned by the end of 8 years? ### Using the Formula for the Sum of an Infinite Geometric Series Thus far, we have looked only at finite series. Sometimes, however, we are interested in the sum of the terms of an infinite sequence rather than the sum of only the first $n n$ terms. An infinite series is the sum of the terms of an infinite sequence. An example of an infinite series is $2+4+6+8+... 2+4+6+8+...$ This series can also be written in summation notation as $∑ k=1 ∞ 2k, ∑ k=1 ∞ 2k,$ where the upper limit of summation is infinity. Because the terms are not tending to zero, the sum of the series increases without bound as we add more terms. Therefore, the sum of this infinite series is not defined. When the sum is not a real number, we say the series diverges. #### Determining Whether the Sum of an Infinite Geometric Series is Defined If the terms of an infinite geometric sequence approach 0, the sum of an infinite geometric series can be defined. The terms in this series approach 0: $1+0.2+0.04+0.008+0.0016+... 1+0.2+0.04+0.008+0.0016+...$ The common ratio As $n n$ gets very large, the values of $r n r n$ get very small and approach 0. Each successive term affects the sum less than the preceding term. As each succeeding term gets closer to 0, the sum of the terms approaches a finite value. The terms of any infinite geometric series with $−1 approach 0; the sum of a geometric series is defined when $−1 ### Determining Whether the Sum of an Infinite Geometric Series is Defined The sum of an infinite series is defined if the series is geometric and $−1 ### How To Given the first several terms of an infinite series, determine if the sum of the series exists. 1. Find the ratio of the second term to the first term. 2. Find the ratio of the third term to the second term. 3. Continue this process to ensure the ratio of a term to the preceding term is constant throughout. If so, the series is geometric. 4. If a common ratio, $r, r,$ was found in step 3, check to see if $−1 . If so, the sum is defined. If not, the sum is not defined. ### Example 6 #### Determining Whether the Sum of an Infinite Series is Defined Determine whether the sum of each infinite series is defined. 1. $3 4 + 1 2 + 1 3 +... 3 4 + 1 2 + 1 3 +...$ 2. $∑ k=1 ∞ 27⋅ ( 1 3 ) k ∑ k=1 ∞ 27⋅ ( 1 3 ) k$ 3. $∑ k=1 ∞ 5k ∑ k=1 ∞ 5k$ Determine whether the sum of the infinite series is defined. ### Try It #9 $1 3 + 1 2 + 3 4 + 9 8 +... 1 3 + 1 2 + 3 4 + 9 8 +...$ ### Try It #10 $24+( −12 )+6+( −3 )+... 24+( −12 )+6+( −3 )+...$ ### Try It #11 $∑ k=1 ∞ 15⋅ (–0.3) k ∑ k=1 ∞ 15⋅ (–0.3) k$ #### Finding Sums of Infinite Series When the sum of an infinite geometric series exists, we can calculate the sum. The formula for the sum of an infinite series is related to the formula for the sum of the first $n n$ terms of a geometric series. $S n = a 1 (1− r n ) 1−r S n = a 1 (1− r n ) 1−r$ We will examine an infinite series with $r= 1 2 . r= 1 2 .$ What happens to $r n r n$ as $n n$ increases? $( 1 2 ) 2 = 1 4 ( 1 2 ) 3 = 1 8 ( 1 2 ) 4 = 1 16 ( 1 2 ) 2 = 1 4 ( 1 2 ) 3 = 1 8 ( 1 2 ) 4 = 1 16$ The value of $r n r n$ decreases rapidly. What happens for greater values of $n? n?$ $( 1 2 ) 10 = 1 1,024 ( 1 2 ) 20 = 1 1,048,576 ( 1 2 ) 30 = 1 1,073,741,824 ( 1 2 ) 10 = 1 1,024 ( 1 2 ) 20 = 1 1,048,576 ( 1 2 ) 30 = 1 1,073,741,824$ As $n n$ gets very large, $r n r n$ gets very small. We say that, as $n n$ increases without bound, $r n r n$ approaches 0. As $r n r n$ approaches 0, $1− r n 1− r n$ approaches 1. When this happens, the numerator approaches $a 1 . a 1 .$ This give us a formula for the sum of an infinite geometric series. ### Formula for the Sum of an Infinite Geometric Series The formula for the sum of an infinite geometric series with $−1 is $S= a 1 1−r S= a 1 1−r$ ### How To Given an infinite geometric series, find its sum. 1. Identify $a 1 a 1$ and $r. r.$ 2. Confirm that $–1 3. Substitute values for $a 1 a 1$ and $r r$ into the formula, $S= a 1 1−r . S= a 1 1−r .$ 4. Simplify to find $S. S.$ ### Example 7 #### Finding the Sum of an Infinite Geometric Series Find the sum, if it exists, for the following: 1. $10+9+8+7+… 10+9+8+7+…$ 2. $248.6+99.44+39.776+… 248.6+99.44+39.776+…$ 3. $∑ k=1 ∞ 4,374⋅ (– 1 3 ) k–1 ∑ k=1 ∞ 4,374⋅ (– 1 3 ) k–1$ 4. $∑ k=1 ∞ 1 9 ⋅ ( 4 3 ) k ∑ k=1 ∞ 1 9 ⋅ ( 4 3 ) k$ ### Example 8 #### Finding an Equivalent Fraction for a Repeating Decimal Find an equivalent fraction for the repeating decimal $0.3¯ 0.3¯$ Find the sum, if it exists. ### Try It #12 $2+ 2 3 + 2 9 +... 2+ 2 3 + 2 9 +...$ ### Try It #13 $∑ k=1 ∞ 0.76k+1 ∑ k=1 ∞ 0.76k+1$ ### Try It #14 $∑ k=1 ∞ ( − 3 8 ) k ∑ k=1 ∞ ( − 3 8 ) k$ ### Solving Annuity Problems At the beginning of the section, we looked at a problem in which a parent invested a set amount of money each month into a college fund for six years. An annuity is an investment in which the purchaser makes a sequence of periodic, equal payments. To find the amount of an annuity, we need to find the sum of all the payments and the interest earned. In the example, the parent invests$50 each month. This is the value of the initial deposit. The account paid 6% annual interest, compounded monthly. To find the interest rate per payment period, we need to divide the 6% annual percentage interest (APR) rate by 12. So the monthly interest rate is 0.5%. We can multiply the amount in the account each month by 100.5% to find the value of the account after interest has been added.

We can find the value of the annuity right after the last deposit by using a geometric series with $a 1 =50 a 1 =50$ and $r=100.5%=1.005. r=100.5%=1.005.$ After the first deposit, the value of the annuity will be $50. Let us see if we can determine the amount in the college fund and the interest earned. We can find the value of the annuity after $n n$ deposits using the formula for the sum of the first $n n$ terms of a geometric series. In 6 years, there are 72 months, so $n=72. n=72.$ We can substitute into the formula, and simplify to find the value of the annuity after 6 years. $S 72 = 50(1− 1.005 72 ) 1−1.005 ≈4,320.44 S 72 = 50(1− 1.005 72 ) 1−1.005 ≈4,320.44$ After the last deposit, the parent will have a total of$4,320.44 in the account. Notice, the parent made 72 payments of $50 each for a total of This means that because of the annuity, the parent earned$720.44 interest in their college fund.

### How To

Given an initial deposit and an interest rate, find the value of an annuity.

1. Determine $a 1 , a 1 ,$ the value of the initial deposit.
2. Determine $n, n,$ the number of deposits.
3. Determine $r. r.$
1. Divide the annual interest rate by the number of times per year that interest is compounded.
2. Add 1 to this amount to find $r. r.$
4. Substitute values for $a 1 ,r,andn a 1 ,r,andn$ into the formula for the sum of the first $n n$ terms of a geometric series, $S n = a 1 (1– r n ) 1–r . S n = a 1 (1– r n ) 1–r .$
5. Simplify to find $S n , S n ,$ the value of the annuity after $n n$ deposits.

### Media

Access these online resources for additional instruction and practice with series.

### 9.4 Section Exercises

#### Verbal

1 .

What is an $nth nth$ partial sum?

2 .

What is the difference between an arithmetic sequence and an arithmetic series?

3 .

What is a geometric series?

4 .

How is finding the sum of an infinite geometric series different from finding the $nth nth$ partial sum?

5 .

What is an annuity?

#### Algebraic

For the following exercises, express each description of a sum using summation notation.

6 .

The sum of terms $m 2 +3m m 2 +3m$ from $m=1 m=1$ to $m=5 m=5$

7 .

The sum from of $n=0 n=0$ to $n=4 n=4$ of $5n 5n$

8 .

The sum of $6k−5 6k−5$ from $k=−2 k=−2$ to $k=1 k=1$

9 .

The sum that results from adding the number 4 five times

For the following exercises, express each arithmetic sum using summation notation.

10 .

$5+10+15+20+25+30+35+40+45+50 5+10+15+20+25+30+35+40+45+50$

11 .

$10+18+26+…+162 10+18+26+…+162$

12 .

$1 2 +1+ 3 2 +2+…+4 1 2 +1+ 3 2 +2+…+4$

For the following exercises, use the formula for the sum of the first $n n$ terms of each arithmetic sequence.

13 .

$3 2 +2+ 5 2 +3+ 7 2 3 2 +2+ 5 2 +3+ 7 2$

14 .

$19+25+31+…+73 19+25+31+…+73$

15 .

$3.2+3.4+3.6+…+5.6 3.2+3.4+3.6+…+5.6$

For the following exercises, express each geometric sum using summation notation.

16 .

$1+3+9+27+81+243+729+2187 1+3+9+27+81+243+729+2187$

17 .

$8+4+2+…+0.125 8+4+2+…+0.125$

18 .

$− 1 6 + 1 12 − 1 24 +…+ 1 768 − 1 6 + 1 12 − 1 24 +…+ 1 768$

For the following exercises, use the formula for the sum of the first $n n$ terms of each geometric sequence, and then state the indicated sum.

19 .

$9+3+1+ 1 3 + 1 9 9+3+1+ 1 3 + 1 9$

20 .

$∑ n=1 9 5⋅ 2 n−1 ∑ n=1 9 5⋅ 2 n−1$

21 .

$∑ a=1 11 64⋅ 0.2 a−1 ∑ a=1 11 64⋅ 0.2 a−1$

For the following exercises, determine whether the infinite series has a sum. If so, write the formula for the sum. If not, state the reason.

22 .

$12+18+24+30+... 12+18+24+30+...$

23 .

$2+1.6+1.28+1.024+... 2+1.6+1.28+1.024+...$

24 .

$∑ m=1 ∞ 4 m−1 ∑ m=1 ∞ 4 m−1$

25 .

$∑ ​ ∞ k=1 − ( − 1 2 ) k−1 ∑ ​ ∞ k=1 − ( − 1 2 ) k−1$

#### Graphical

For the following exercises, use the following scenario. Javier makes monthly deposits into a savings account. He opened the account with an initial deposit of $50. Each month thereafter he increased the previous deposit amount by$20.

26 .

Graph the arithmetic sequence showing one year of Javier’s deposits.

27 .

Graph the arithmetic series showing the monthly sums of one year of Javier’s deposits.

For the following exercises, use the geometric series $∑ k=1 ∞ ( 1 2 ) k . ∑ k=1 ∞ ( 1 2 ) k .$

28 .

Graph the first 7 partial sums of the series.

29 .

What number does $S n S n$ seem to be approaching in the graph? Find the sum to explain why this makes sense.

#### Numeric

For the following exercises, find the indicated sum.

30 .

$∑ a=1 14 a ∑ a=1 14 a$

31 .

$∑ n=1 6 n(n−2) ∑ n=1 6 n(n−2)$

32 .

$∑ k=1 17 k 2 ∑ k=1 17 k 2$

33 .

$∑ k=1 7 2 k ∑ k=1 7 2 k$

For the following exercises, use the formula for the sum of the first $nn$ terms of an arithmetic series to find the sum.

34 .

$−1.7+−0.4+0.9+2.2+3.5+4.8 −1.7+−0.4+0.9+2.2+3.5+4.8$

35 .

$6+ 15 2 +9+ 21 2 +12+ 27 2 +15 6+ 15 2 +9+ 21 2 +12+ 27 2 +15$

36 .

$−1+3+7+...+31 −1+3+7+...+31$

37 .

$∑ k=1 11 ( k 2 − 1 2 ) ∑ k=1 11 ( k 2 − 1 2 )$

For the following exercises, use the formula for the sum of the first $n n$ terms of a geometric series to find the partial sum.

38 .

$S 6 S 6$ for the series $−2−10−50−250... −2−10−50−250...$

39 .

$S 7 S 7$ for the series $0.4−2+10−50... 0.4−2+10−50...$

40 .

$∑ k=1 9 2 k−1 ∑ k=1 9 2 k−1$

41 .

$∑ n=1 10 −2⋅ ( 1 2 ) n−1 ∑ n=1 10 −2⋅ ( 1 2 ) n−1$

For the following exercises, find the sum of the infinite geometric series.

42 .

$4+2+1+ 1 2 ... 4+2+1+ 1 2 ...$

43 .

$−1− 1 4 − 1 16 − 1 64 ... −1− 1 4 − 1 16 − 1 64 ...$

44 .

$∑ ​ ∞ k=1 3⋅ ( 1 4 ) k−1 ∑ ​ ∞ k=1 3⋅ ( 1 4 ) k−1$

45 .

$∑ n=1 ∞ 4.6⋅ 0.5 n−1 ∑ n=1 ∞ 4.6⋅ 0.5 n−1$

For the following exercises, determine the value of the annuity for the indicated monthly deposit amount, the number of deposits, and the interest rate.

46 .

Deposit amount: $50; 50;$ total deposits: $60; 60;$ interest rate: $5%, 5%,$ compounded monthly

47 .

Deposit amount: $150; 150;$ total deposits: $24; 24;$ interest rate: $3%, 3%,$ compounded monthly

48 .

Deposit amount: $450; 450;$ total deposits: $60; 60;$ interest rate: $4.5%, 4.5%,$ compounded quarterly

49 .

Deposit amount: $100; 100;$ total deposits: $120; 120;$ interest rate: $10%, 10%,$ compounded semi-annually

#### Extensions

50 .

The sum of terms $50− k 2 50− k 2$ from $k=x k=x$ through $7 7$ is $115. 115.$ What is x?

51 .

Write an explicit formula for $a k a k$ such that $∑ k=0 6 a k =189. ∑ k=0 6 a k =189.$ Assume this is an arithmetic series.

52 .

Find the smallest value of n such that $∑ k=1 n (3k–5)>100. ∑ k=1 n (3k–5)>100.$

53 .

How many terms must be added before the series $−1−3−5−7.... −1−3−5−7....$ has a sum less than $−75? −75?$

54 .

Write $0. 65 ¯ 0. 65 ¯$ as an infinite geometric series using summation notation. Then use the formula for finding the sum of an infinite geometric series to convert $0. 65 ¯ 0. 65 ¯$ to a fraction.

55 .

The sum of an infinite geometric series is five times the value of the first term. What is the common ratio of the series?

56 .

To get the best loan rates available, the Coleman family want to save enough money to place 20% down on a $160,000 home. They plan to make monthly deposits of$125 in an investment account that offers 8.5% annual interest compounded semi-annually. Will the Colemans have enough for a 20% down payment after five years of saving? How much money will they have saved?

57 .

Karl has two years to save $10,000 10,000$ to buy a used car when he graduates. To the nearest dollar, what would his monthly deposits need to be if he invests in an account offering a 4.2% annual interest rate that compounds monthly?

#### Real-World Applications

58 .

Keisha devised a week-long study plan to prepare for finals. On the first day, she plans to study for $1 1$ hour, and each successive day she will increase her study time by $30 30$ minutes. How many hours will Keisha have studied after one week?

59 .

A boulder rolled down a mountain, traveling 6 feet in the first second. Each successive second, its distance increased by 8 feet. How far did the boulder travel after 10 seconds?

60 .

A scientist places 50 cells in a petri dish. Every hour, the population increases by 1.5%. What will the cell count be after 1 day?

61 .

A pendulum travels a distance of 3 feet on its first swing. On each successive swing, it travels $3 4 3 4$ the distance of the previous swing. What is the total distance traveled by the pendulum when it stops swinging?

62 .

Rachael deposits \$1,500 into a retirement fund each year. The fund earns 8.2% annual interest, compounded monthly. If she opened her account when she was 19 years old, how much will she have by the time she is 55? How much of that amount will be interest earned?