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Imagine that a set of numbers is arranged according to a certain rule, just like friends with unique qualities forming a line. For example, consider a set of numbers where each one is $3$ more than the one before.
*sequences*. Exploring sequences is important for understanding the order and connections in numbers, which comes in handy when solving different math problems. ### Catch-Up and Review

$1,4,7,10,13,… $

Another example can be the following ordered numbers where each number is the sum of the two before it. $1,1,2,3,5,… $

These arrangements that follow a specific rule are called **Here are a few recommended readings before getting started with this lesson.**

Explore

Interact with the applet and try to identify a pattern between the figures. What would Figure $4$ look like?

Is there an expression to represent the number of points in terms of the figure's number?

Discussion

A sequence is an ordered list of objects or elements called terms. The terms are often represented using a variable labeled with indices that specify the positions of the terms in the sequence.

Depending on the number of terms, a sequence can be finite or infinite. Since it is not possible to list all the elements in an infinite sequence, it is common to place three dots after a few terms. These three dots indicate that the sequence continues indefinitely based on a specific pattern.

The values of the terms of a sequence can repeat.

$Sequence With Repeating Termsa_{1}-1, a_{2}0, a_{3}1, a_{4}0, a_{5}-1, a_{6}0, a_{7}1, … $

Sequences can have all sorts of patterns. The examples below use the same starting term but result in different sequences due to the differences in the patterns. Note that it is common to use letters like $a_{n},$ $b_{n},$ $c_{n},$ and so on to represent distinct sequences. Discussion

A pattern describes a repeated change of numbers, shapes, colors, actions, or other elements. Patterns are based on a specific rule. The rule can also be used to find missing steps in the pattern. In the example below, matches have been placed together to create three figures.

Is it possible to find a pattern? Notice that each figure has one more triangle than the previous one. Therefore, the next figure should have $4$ triangles.

There is also a pattern in the number of matches. In each step, the number increases by $2.$ The first figure has $3$ matches, the second figure has $5$ matches, the third figure has $7$ matches, and so on.

The number of matches in the next couple of figures can be found using this pattern.

$3,5,7,9,11,13,… $

Example

Emily and Kevin heard whispers of a mythical library filled with magical books in their town. Their research led them to some books full of mysterious numbers left behind by earlier explorers.

a Emily and Kevin came across the following numbers on one page of a book.

$2,9,16,23,… $

b On another page there were the following numbers.

$3,12,48,192,… $

Help them identify the pattern and find the next three terms for each sequence.
a **Pattern:** Previous term plus $7$

**Next Three Terms:** $30,$ $37,$ and $44$

b **Pattern:** Previous term times $4$

**Next Three Terms:** $768,$ $3072,$ and $12288$

a How is one term related to the next one?

b How is one term related to the next one?

As shown, each subsequent term can be found by $adding$ $7$ to the previous one.

$Pattern:Previous term plus7 $

Now use this pattern to find the next three terms in the sequence.
The next three terms of the sequence are $30,$ $37,$ and $44.$

$Next Three Terms:30,37,and44 $

b Examine the given sequence.

In this case, there is no common difference between consecutive terms. In a case like this, the next step is to check whether the terms are multiples of each other.

In this sequence, each term can be found by $multiplying$ the previous term $by$ $4.$

$Pattern:Previous term times4 $

Now use this pattern to find the next three terms in the sequence.
The next three terms of the sequence are $768,$ $3072,$ and $12288.$

$Next Three Terms:768,3072,and12288 $

Could these numbers represent the location of the library? Kevin and Emily continue to search for clues.
Pop Quiz

Discussion

Sequences are ordered sets of numbers that follow identifiable patterns. Two types of sequences are *arithmetic* and *geometric*, each with their own unique characteristics. They play a key role in understanding the organized relationships between numbers.

Concept

An arithmetic sequence is a sequence that has a constant difference between consecutive terms — that is, the difference between the first and the second term is the same as the difference between the second and the third term, and so on. This difference is called the common difference and is usually denoted with $d.$ For example, consider the sequence of all even positive integers

For this sequence, the common difference is $d=2.$ It is important to note that the common difference can take on negative values as well. Consider the following arithmetic sequence where the values decrease.

This is an arithmetic sequence with a common difference of $-3.$

Discussion

Unlike the adding pattern in arithmetic sequences, geometric sequences multiply each term by a constant factor. The characteristics of geometric sequences will now be explored.

Concept

A geometric sequence is a sequence in which the ratio $r$ between consecutive terms is a nonzero constant. This ratio is called the common ratio. The following is an example geometric sequence with first term $3$ and common ratio $2.$

The behavior of geometric sequences generally depends on the values of the first term $a_{1}$ and the common ratio $r.$ The following table shows the effects of these parameters.

$a_{1}>0$ | $a_{1}<0$ | |
---|---|---|

$r>1$ | Increasing $3→×26→×212→×224→×248…$ |
Decreasing $-3→×2-6→×2-12→×2-24→×2-48…$ |

$r=1$ | Constant
$3→×13→×13→×13→×13…$ |
Constant
$-3→×1-3→×1-3→×1-3→×1-3…$ |

$0<r<1$ | Decreasing $48→×21 24→×21 12→×21 6→×21 3…$ |
Increasing $-48→×21 -24→×21 -12→×21 -6→×21 -3…$ |

$r<0$ | Alternating
$3→×(-2)-6→×(-2)12→×(-2)-24→×(-2)48…$ |
Alternating
$-3→×(-2)6→×(-2)-12→×(-2)24→×(-2)-48…$ |

Example

After a while, Izabella joined Kevin and Emily on their adventure. To find the mysterious library, they decided to challenge themselves by reading the books as quickly as possible.

They each had their own reading goals. Emily aimed to increase her reading by $15$ pages each day, Kevin planned to read double the pages of the previous day, and Izabella alternated between reading $15$ and $25$ pages every day.

a Emily read $15$ pages on the first day. Classify the pattern of the pages Emily read as arithmetic, geometric, or neither.

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b Izabella read $15$ pages on the first day. Classify the pattern of the pages Izabella read as arithmetic, geometric, or neither.

{"type":"choice","form":{"alts":["Arithmetic Sequence","Geometric Sequence","Neither"],"noSort":false},"formTextBefore":"","formTextAfter":"","answer":2}

c Kevin read $5$ pages on the first day. Classify the pattern of the pages Kevin read as arithmetic, geometric, or neither.

{"type":"choice","form":{"alts":["Arithmetic Sequence","Geometric Sequence","Neither"],"noSort":false},"formTextBefore":"","formTextAfter":"","answer":1}

b How many pages did Izabella read on the first, second, third, and fourth days?

c How many pages did Kevin read on the first, second, third, and fourth days?

a Emily read $15$ pages on the first day and then increased her daily reading by an additional $15$ pages each day after that. With this information, the number of pages she read in the first four days can be shown in a table as follows.

Since there is a common difference between consecutive terms, the number of pages read forms an *arithmetic sequence*.

Sequence | Classification |
---|---|

$15,30,45,60,…$ | Arithmetic Sequence |

b It is given that Izabella read $15$ pages on the first day and then alternated between reading $15$ and $25$ pages every day. With this information in mind, the number of pages she read in the first four days can be found using a table.

In this case, there is no common difference or common ratio between consecutive terms, so the number of pages is *neither* arithmetic nor geometric.

Sequence | Classification |
---|---|

$15,25,15,25,…$ | Neither arithmetic nor geometric |

c Finally, take a look at the sequence formed by the number of pages Kevin read. He read $5$ pages the first day, then $doubled$ the number of pages he read every day.

Since there is a common ratio between consecutive terms, the number of pages read forms a *geometric sequence*.

Sequence | Classification |
---|---|

$5,10,20,40,…$ | Geometric Sequence |

Pop Quiz

The following applet shows the first five terms of an infinite sequence. Analyze them carefully and determine whether the sequence is arithmetic, geometric, or neither.

Example

As Emily, Kevin, and Izabella continued their search, the books revealed more clues leading them closer to the magical library. They noticed that the mysterious numbers left by previous adventurers seemed to follow distinct patterns.

Find the fifth term of each sequence, then use the first five terms to graph each sequence on a coordinate plane.

a $7,14,21,28,…$

b $3,9,27,81,…$

a **Fifth Term:** $35$

**Graph:**

b **Fifth Term:** $243$

**Graph:**

a To find the fifth term, determine if the sequence is arithmetic or geometric. A sequence $a_{n}$ can be considered a function of its position $n.$ Make a table by pairing each term with its position, then plot the ordered pairs $(n,a_{n})$ on the coordinate plane.

b Is the sequence arithmetic or a geometric? Make a table by pairing each term with its position. Then, plot the ordered pairs $(n,a_{n})$ on the coordinate plane.

a A sequence $a_{n}$ can be considered a function of its position $n.$ Recall that a term position takes whole number values. The terms of the given sequence can be organized in a table as follows.

In this sequence, the difference between consecutive terms is $7,$ so it is an arithmetic sequence with a common difference of $7.$ This means that the fifth term of the sequence is calculated by adding $7$ to the fourth term.

To graph this sequence, start by drawing a coordinate plane where the horizontal axis represents the position $n$ and the vertical axis represents the term $a_{n}.$ Then, plot the ordered pairs $(n,a_{n})$ on the coordinate plane.

b Follow the same procedure as in Part A. First, make a table to show the terms of the given sequence.

In this sequence, the difference between the first and second terms is $6,$ but the difference between the second and third terms is $18.$ There is no common difference between consecutive terms. However, there is a common ratio between consecutive terms — $3$ — so this is a geometric sequence. The fifth term of this sequence will be $3$ times the fourth term.

To graph this sequence, start by drawing a coordinate plane where the horizontal axis represents the position $n$ and the vertical axis represents the term $a_{n}.$ Then, plot the ordered pairs $(n,a_{n})$ on the coordinate plane.

Looking over the completed graphs, Izabella said, The arithmetic sequence looks like a straight line, but the geometric sequence looks like a curve. I wonder if this will be important.

Discussion

Every arithmetic sequence can be described by a linear function that is defined for the set of counting numbers. This function, referred to as the explicit rule of an arithmetic sequence, follows a specific general format.

$a_{n}=a_{1}+(n−1)d$

Every arithmetic sequence has a common difference $d.$ Therefore, it is possible to obtain every term of the sequence by adding the common difference to the first term $a_{1}$ an appropriate number of times.

Tables can help in identifying the pattern and writing a general expression.

$n$ | $a_{n}$ | Rewrite |
---|---|---|

$1$ | $a_{1}$ | $a_{1}+0⋅d$ |

$2$ | $a_{2}$ | $a_{1}+1⋅d$ |

$3$ | $a_{3}$ | $a_{1}+2⋅d$ |

$4$ | $a_{4}$ | $a_{1}+3⋅d$ |

$5$ | $a_{5}$ | $a_{1}+4⋅d$ |

The coefficient of the common difference is always $1$ less than the value of the position $n.$ This makes it possible to write an explicit rule like the following formula.

$a_{n}=a_{1}+(n−1)d$

A sequence can be thought of as a set of coordinate pairs where the first coordinate is the position $n$ and the second coordinate is the term value $a_{n}.$

$(1,a_{1}),(2,a_{2}),(3,a_{3}),… $

As the position increases by $1,$ the value of the term increases, or decreases, by a constant. Therefore, the rate of change between two consecutive coordinate pairs is constant and equal to $d.$ That means an arithmetic sequence is a linear function with a slope $d.$
Therefore, the explicit rule for the sequence can be written by substituting the coordinate pair $(1,a_{1})$ into the point-slope form of a line.
$Point-Slope Formy−y_{1}=m(x−x_{1})Explicit Rulea_{n}−a_{1}=d(n−1) $

Finally, the explicit rule can be rewritten to the form given at the beginning of this proof.
$a_{n}−a_{1}=d(n−1)⇕a_{n}=a_{1}+(n−1)d $

Example

In the heart of their town, the three friends finally reached the entrance of the mystical library.

All the mysteries lay behind a door with symbols on it. To unlock the door, they had to solve one more puzzle. Help them open the door.

a Write the explicit rule for the sequence.

$12,20,28,36,… $

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b Find the $8th$ term of the sequence to help Kevin, Izabella, and Emily unlock the next chapter of their adventure.

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a Is the sequence an arithmetic sequence? If so, identify its first term and common difference.

b Substitute $8$ for $n$ into the explicit rule.

a For an arithmetic sequence, the difference between consecutive terms is constant. Check if this is true for the given sequence.

$a_{n}=a_{1}+(n−1)d⇓a_{n}=12+(n−1)8 $

Distribute $8$ to simplify the explicit rule.