Algebra 2/Trig: Chapter 6 – Sequences and Series

In this unit, we will…

 Identify an arithmetic or geometric sequence and find the formula for its nth term
 Determine the common difference in an arithmetic sequence
 Determine the common ratio in a geometric sequence
 Determine a specified term of an arithmetic or geometric sequence
 Specify terms of a sequence, given its recursive definition
 Represent the sum of a series, using sigma notation
 Determine the sum of the first n terms of an arithmetic or geometric series
 Table of Contents
Day 1: Arithmetic and Geometric Sequences
SWBAT:
 Identify an arithmetic or geometric sequence and find the formula for its nth term
 Determine the common difference in an arithmetic sequence
 Determine the common ratio in a geometric sequence
 Determine a specified term of an arithmetic or geometric sequence
 Specify terms of a sequence, given its recursive definition

Pgs. 1 – 6 in Packet

HW: Pgs. 7 – 9 in Packet

Day 2: More with Arithmetic & Geometric Sequences

SWBAT:
 Identify an arithmetic or geometric sequence and find the formula for its nth term
 Determine the common difference in an arithmetic sequence
 Determine the common ratio in a geometric sequence
 Determine a specified term of an arithmetic or geometric sequence
 Specify terms of a sequence, given its recursive definition

Pgs. 10 – 14 in Packet

HW: Pgs. 15– 17 in Packet

QUIZ on Day 3 ~ 7 min

Day 3: Series
SWBAT:
 Represent the sum of a series, using sigma notation
 Determine the sum of the first n terms of an arithmetic or geometric series

Pgs. 18 – 22 in Packet

HW: Pgs. 23 – 27 in Packet

Day 4: Sequences and Series Mixed Practice

SWBAT: Review problems involving Sequences and Series
Pgs. 28 – 32 in Packet

QUIZ on Day 4 ~ 15 min

Day 5: Practice Test (Not in Packet)

Day 6: Test
 Lesson #1: Arithmetic and Geometric Sequences

Definition of a sequence:

Example 1:
Example 2:

Concept 1: Ways to define a sequence

There are two ways to define a sequence: _____________or ________________.

An explicitly defined sequence is like a formula. Plugging into the formula gives
the terms of the sequence.

 Subscripts name terms. They are not values in the problem.

Example 3: Consider = 3n + 2. Find the first 3 terms ( ) of this

sequence.

A recursively defined sequence has two parts;

(1) It gives the first term and
(2) all of the other terms are found using operations on the previous term(s)

Key Points for Recursive Formulas

 Subscripts name terms. They are not values in the problem.
 If the next term is an the term before it will be an 1 because n-1 is one smaller than n.

For example, if
an is a3 , an 1 is a2 .
 If the next term is an 1 the term before it will be an because n is one smaller than n+1.

For example, if
an 1 is a3 , an is a2 .
Bottom Line: Build off the last term!
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For each problem, find the next four terms.

Example 4: Example 5:
a1  4 a1  5
an 1   an   10 an  an1  n
2

Label the following as either an Explicit Formula or a Recursive Formula.

2
 Arithmetic Sequences

If a sequence of values follows a pattern of adding a fixed amount from one term to the next,
it is referred to as an arithmetic sequence. The number added to each term is constant
(always the same). The fixed amount is called the common difference, d.

a) The following is an example of an arithmetic sequence: 3,8,13,18,23 . . .

What is the common difference, d?

7 1
b) What is the common difference of the following arithmetic sequence?: 5, , 2, , . . .
2 2

Geometric Sequences
If a sequence of values follows a pattern of multiplying a fixed amount (not zero) times each
term to arrive at the following term, it is referred to as a geometric sequence. The number
multiplied each time is constant (always the same). The fixed amount is called the common
ratio, r.

a) The following is an example of a geometric sequence: 8,56,392, 2744 . . .

What is the common ratio, r?

b) What is the common ratio of the following geometric sequence?: 27, 9, 3, 1, ,...

Concept 2: Generating a Sequence

A sequence can be defined by a formula (or generator) which generates each
term. (Note: the variable “n” appears in most generator. It is used to indicate the position of a term in a
sequence.)

 The formula to generate any arithmetic sequence can be written in

the form:

 The formula to generate any geometric sequence can be written in

the form:
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Example 6: Find a formula to generate the arithmetic sequence 3, 5, 7, … and use it to
generate the 50th term.

Step 1:

Step 2:

Example 7: Find a formula to generate the geometric sequence 4, 12, 36, … and use it to
determine the 19th term.

Step 1:

Step 2:

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You Try it!

Determine if the sequence is arithmetic. If it is, find the common difference, the term
named in the problem, and the explicit formula.

Determine if the sequence is geometric. If it is, find the common ratio, the term named in
the problem, and the explicit formula.

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SUMMARY

Exit Ticket
1.

2.

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7
8
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 Lesson #2: More with Arithmetic & Geometric Sequences
WARM-UP!
Writing Sequences using equations practice

1) an  3n  2 What type of sequence is this? _________________________

Write the first 5 terms of the sequence: _________________________________________________

n 1
1
an  162 
 3
2) What type of sequence is this? _________________________

Write the first 4 terms of the sequence: _________________________________________________

3) an  3(2)n What type of sequence is this? _________________________

Write the first 5 terms of the sequence: _________________________________________________

1
4)
an  6  ( n  1) What type of sequence is this? _________________________
2
Write the first 5 terms of the sequence: _________________________________________________

5) a1  1 and the common difference of this arithmetic sequence is -3.

Write the first 4 terms of the sequence:____________________________________

Write the explicit equation of this sequence:_________________________________

6) a1  2 and the common ratio of this geometric sequence is 10.

Write the first 4 terms of the sequence:____________________________________

Write the explicit equation of this sequence:_________________________________

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 9 4 4
7) Write an explicit equation for the sequence:
, 3, 2, , , ...
2 3 9

8) Write an explicit equation for the sequence: -7.8, -5.6, -3.4, -1.2, . . .

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Concept 1: Arithmetic Mean

Arithmetic mean- the mean average between any two numbers of a sequence

-a missing term can be found by finding the arithmetic mean of two terms.

Ex: Given the arithmetic sequence 84,____, 110 , find the missing term.

Arithmetic mean =

= 97

Ex: Find the missing term of each arithmetic sequence.

1) 16, _____, 36

2) 23, _____, _____, 17

3) 12, ______,______,______, 0

4) If = 80 and = 32 in a arithmetic sequence, find the 24th term.

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 Concept 2: Geometric Mean
Geometric mean- the positive square root of the product of two numbers of a sequence

- A missing term can be found by finding the geometric mean of two terms

Ex: Given: 20, _____, 80

Step 1:
Step 2:
Step 3:

Find the missing term of each geometric sequence.

1) 3, ___ , 18.75

2) 28, ____, ____, 9604

3) 19,683, ______, ______ , ______, 243

4) If = -6 and = -1296 in a geometric sequence, find the 14th term.

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SUMMARY

Exit Ticket

1)

2)

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 HW – Day 2

Two important types of sequences are arithmetic sequences and

geometric sequences. Try to figure out which is which, and fill in the
missing number in each sequence.
arithmetic or geometric

1) 4, 7, 10, _____, 16, . . . _____ _____

2) 2, 4, 8, 16, _____, . . . _____ _____

3) 1, _____, 9, 27, 81, . . . _____ _____

4) 3.5, 6, 8.5, 11, _____, . . . _____ _____

5) 8, 12, 18, _____, 40.5, . . . _____ _____

6) _____, -5.5, -9.5, -13.5, . . . _____ _____

7) 256, 64, 16, 4, _____, . . . _____ _____

8) -4, 8, -16, 32, -64, _____, . . . _____ _____

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16
Day 2 - Answers

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 Day 3 – Series

Review of Summation Notation

Last value for the index variable
5 Plug the values into this
Symbol that tells us to add

Index variable (the one we

k
k 2
expression and add up each term

plug the values into) First value for the index variable

 Find each sum.

1. 4

a
a 1
2
 2a

2.

A series is the sum of the terms in a sequence. On the first page of this lesson, you reviewed
the different ways we have used summation notation so far this year.

1. Which of the following represents the sum 8+12+16+20+24?

4 4

8  a
a 0
(2)  8  4(a  1)
a 1
(1)

5 4

 8  4a
a 0
 8  4a
a 0
(3) (4)

Solution:

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You try it!
2. Which of the following represents the sum 3+8+15+24+ . . . 80?
9 9

(1)  3  5(n  1)
n 1
(2) n
n2
2
1

9 80

(3) n
n 1
2
1 (4) n
n 3

3.

Imagine having to find the sum of the arithmetic series 3+6+9+ . . . 129+132 by hand.
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Imagine having to find the sum of the geometric series  4(3)

k 1
k 1
by hand.

Luckily there are formulas to find the sum of the first n terms of any arithmetic or geometric
series. Even more luckily, you do not have to memorize them because they are given to you on
the A2&T reference sheet.

The rest of this lesson will deal with some problems you might encounter where you need to use
these formulas. You will also need to use the other formulas from the previous lessons to find
the numbers to plug into the formulas.

Finding the nth term in an arithmetic sequence:

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Finding the nth term in a geometric sequence:
1. Find the sum of the first 28 terms of the series 3 + 6 + 9 + 12 + ...
(Hint: You need to find a28 first).

2. Find the sum of the first 12 terms of the series, -3 + 6 - 12 + 24 - 48 + ...

(Since this one is geometric, we do not need a12 )

3.
Evaluate this series using the series formula:

4.
Evaluate using the series formula:

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5.

6.

7.

8. Find the sum of the first 20 terms of the sequence 4, 6, 8, 10, ...

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9. Find the sum of the sequence -8, -5, -2, ..., 7

10. Find the sum of the first 8 terms of the sequence -5, 15, -45, 135, ...

SUMMARY

Exit Ticket

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 Day 3 – Series HW

Sum of Arithmetic Series Name______________________

n(a1  an )
Sum of a Finite Arithmetic Series:
Sn 
2
1) Evaluate each arithmetic series.

a) a1  4, an  22, n  10 b) a1  2, an  156, n  10

100 50
c)  (2n  1)
n 1
d)  (3n  2)
n 1

e) 20, 23, 26, 29, . . . n=25 f) 20, 30, 40, 50, . . . n=30

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2) Determine the number of terms n in the following arithmetic series.

a) a1  19, an  96, Sn  690 b) a1  15, an  79, Sn  423

3) Determine the sum of the all of the even integers from 2 to 2000.

4) A company offers Sue a starting salary of $50,000 plus a guaranteed pay increase of
5,000 each year. What is the total amount of money Sue will have earned after working
for 25 years?

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Sum of Geometric Series Name______________________

a1 (1  r n )
Sum of a Finite Geometric Series:
Sn 
1 r
1) Evaluate each geometric series.

7 12
a)  (4
k 1
k 1
) b)  43
n 1
n 1

c) 1, -4, 16, -64, . . . n=9 d) 1, 2, 4, 8, . . . n=30

e) a1  4, an  8748, r  3 f) a1  4, an  1024, r  2

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2) Determine the number of terms n in the following geometric series.

a) a1  2, r  5, Sn  62 b) a1  3, r  3, Sn  60

3) A company offers Jim a starting salary of $50,000 plus a guaranteed pay increase of
5% each year. What is the total amount of money Jim will have earned after working
for 25 years?

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Answers to Arithmetic Series Answers to Geometric Series

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 Day 4 - Sequences and Series Mixed Practice 2014
1. What is the difference between an arithmetic and a geometric sequence?

2. Find the next three terms of each sequence

a. 9, 16, 23, ________, ________, ________

b. 100, -200, 400, ________, ________, ________

c. -8, -5, -2, ________, ________, ________

3. Find the first three terms of each sequence where d is the common difference, and r is the common ratio
1
a. a1  576 , r   ________, ________, ________
2

b. a1  2, d  13 ________, ________, ________

5 3
c. a1  ,d  ________, ________, ________
8 8

4. Find a 8 if an  4  3n .

n1
1
5. Find a 7 if an  12  .
2

6. Find a12 for -17, -13, -9, …

7. Find a 8 for 4, -12, 36, …

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8. Find a14 if a1  3 and the common difference is d 7

1
9. Find a 8 if the common ratio r  3 and a1 
3

10. Write the equation for the nth term for each sequence

a. 7, 16, 25, 34, …

b. 36, 12, 4, …

11. Find the 10th term of the arithmetic sequence given the following information: a3  55, a7  115

12. Find the 7th term of the geometric sequence given the following information: a1  9, a5  144

Write the first 5 terms of each sequence described below with recursive equations:
a1  3
13.
an1  3an  10

a1  5
14.
an  an1  4

a1  2
15. a 2  1
a n  2  a n 1  4a n

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Series Questions

Find the value of each series below.

3
16.  ( 2 j  6)
j 1

17.  2( 4)
k 0
k

18. 6 j j
j 1

19. Find 2  x k if x3  2 , x4  4 , x5  8 and x6  10

k 3

20. Write this series using sigma notation: 2  9  16

21. Write this series using sigma notation: 3  12  48  192  768

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Evaluate each arithmetic series.
22. a1  2, a8  33 , S8  ?

50
23.  (7n  1)
n 1

Evaluate each geometric series.

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24. 53
n 1
n 1

25. 1, 2, 4, 8, . . . n = 25

26. Determine the number of terms n in the following geometric series.

a1  2, r  5, Sn  62

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27) Expand the following binomials. (Remember to fully simplify each term.)

a) ( x  3)5

b) (a  2)6

c) (3x  2 y)3

d) Find the 3rd term of (2a2  3 y)7 .

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