Sequences
Sequences in math are a list of elements (numbers most of the time) in which the order of elements matters. Here, the elements follow a specific pattern. We come across sequences in many places in real life. For example, the house numbers in a row, salary in successive years (by a fixed amount or a by a fixed percentage), page numbers of a book, etc represent sequences.
Let us learn more about sequences along with their types, rules, formulas, and examples.
What is a Sequence?
A sequence is a list of numbers (or elements) that exhibits a particular pattern. For example, Olivia has been offered a job with a starting monthly salary of $1000 with an annual increment of $500. Can you calculate her monthly salary in the first three years? It will be $1000, $1500, and $2000. Observe that Olivia's salary over a number of years forms a sequence as they follow a pattern where the numbers are increasing by an amount of $500 every time.
Order of the Sequence
The order of a sequence can be either in ascending order or in descending order.
Ascending Order
If the elements of the sequence are in increasing order, then the order of the sequence is ascending.
The above sequence is in ascending order as its terms are "increasing" by 2.
Descending Order
If the elements of the sequence are in decreasing order, then the order of the sequence is descending.
The above sequence is in descending order as its terms are "decreasing" by 4.
Finite and Infinite Sequences
There are two types of sequences: finite sequences and infinite sequences. It is possible to count the number of terms in a finite sequence. An infinite sequence is a sequence that is not finite.
Finite Sequence
A sequence having a finite number of terms is called a finite sequence. For example, a sequence of the number of bounces a ball takes to come to the rest is a finite sequence.
Infinite Sequence
A sequence having an infinite number of terms is called an infinite sequence. For example, a sequence of natural numbers forms an infinite sequence: 1, 2, 3, 4, and so on.
Types of Sequences in Math
There are a few special sequences like arithmetic sequence, geometric sequence, Fibonacci sequence, harmonic sequence, triangular number sequence, square number sequence, and cube number sequence. Apart from these, there can be sequences that follow some other pattern. For example, 2, 9, 28, 65, ... is a sequence in which the numbers can be written as 1^{3} + 1, 2^{3} + 1, 3^{3} + 1, 4^{3} + 1,.... and this sequence does not belong to any of the following sequences.
We will discuss these sequences in detail.
Arithmetic Sequence
An arithmetic sequence is a sequence of numbers in which each successive term is a sum of its preceding term and a fixed number. This fixed number is called a common difference. The terms of the arithmetic sequence are of the form a, a+d, a+2d, ....
Example: Mushi put $30 in her piggy bank when she was 7 years old. She increased the amount on her each successive birthday by $3. So, the amount in her piggy bank follows the pattern of $30, $33, $36, and so on. The succeeding terms are obtained by adding a fixed number, that is, $3. This fixed number is called a common difference. It can be positive, negative, or zero.
Quadratic Sequence
We have already seen that if the differences (referred to as first differences) between every two successive terms are the same, then it is called an arithmetic sequence (which is also known as a linear sequence). But if the first differences are NOT same, and instead, the second differences are the same, then the sequence is known as a quadratic sequence.
Example: The sequence 1, 2, 4, 7, 11, ... is a quadratic sequence because their second differences are the same. Take a look at the figure below.
Geometric Sequence
A geometric sequence is a sequence where every term bears a constant ratio to its preceding term. This ratio is called the "common ratio". The terms of the geometric sequence are of the form a, ar, ar^{2}, ....
Example: Consider an example of geometric sequence: 1, 4, 16, 64, .... Here, 4/1 = 16/4 = 64/4 = ... = 4. Hence, it is a geometric sequence with common ratio 4.
Harmonic Sequence
A harmonic sequence is a sequence obtained by taking the reciprocal of the terms of an arithmetic sequence.
Example: We know that the sequence of natural numbers is an arithmetic sequence. So, taking reciprocals of each term, we get 1, 1/2, 1/3, ..., which is a harmonic sequence as their reciprocals 1, 2, 3, ... form an arithmetic sequence.
Triangular Number Sequence
A triangular number sequence is a sequence that is obtained from a pattern forming equilateral triangles. Look at the figure below.
The sequence 1, 3, 6, 10, and so on is a triangular number sequence.
Square Number Sequence
A square number sequence is a sequence that is obtained from a pattern forming squares. Look at the figure below.
The sequence 1, 4, 9, 16, and so on is a square number sequence.
Cube Number Sequence
A cube number sequence is a sequence that is obtained from a pattern forming cubes. Look at the figure below.
The sequence 1, 8, 27, 64, and so on is a cube number sequence.
Fibonacci Sequence
Fibonacci sequence is a sequence where every term is the sum of the last two preceding terms.
Example: A pair of rabbits do not reproduce in their 1st month. Starting from the 2nd month and every subsequent month, they reproduce another pair. Thus, the number of rabbits starting from 1st month are 0, 1, 1, 2, 3, 4, 7, 11, .... This is called the Fibonacci sequence.
Series and Partial Sums of Sequences
Consider a sequence given by a_{1}, a_{2}, a_{3}, a_{4}, .... Then the sum a_{1 }+ a_{2 }+ a_{3} + a_{4} + .... is the series associated with the sequence. Series can be represented using sigma notation, ∑. So, the series is represented as ∑_{n=1}^{∞} a_{n}. The partial sum is a part of the series. The sum up to k terms in the series ∑_{n=1}^{k} a_{n }and it is called the partial sum of the series. We can find the differences between a sequence and a series by clicking here.
Example: Consider a sequence of prime numbers: 2, 3, 5, 7, 11, and so on. The series associated with this is ∑_{n=1}^{∞} a_{n}, where a_{n} is the n^{th} prime number. The partial sum up to 4 terms is 2+3+5+7=17.
Rules of Sequences
We can generally have two types of rules for a sequence (if it is geometric/arithmetic).
 Implicit rule where a term is expressed in terms of its previous term.
 Explicit rule where any term can be found using a general formula.
Example: Consider the sequence of odd numbers 3, 5, 7, .... We will define two rules to define n^{th} term (general term) of this sequence. Note that this sequence is an arithmetic sequence with the first term 3 (a = 3) and the common difference 2 (d = 5  3 = 2). Then:
 Implicit rule: a_{n} = a_{n1} + 2
 Explicit rule: a_{n} = a + (n  1) d = 3 + (n  1) 2 = 3 + 2n  2 = 2n + 1.
Here, a_{n} = a + (n  1) d is the formula for nᵗʰ term of the arithmetic sequence. Let us see more formulas of different types of sequences in the upcoming section.
Formulas of Sequences
As we have seen in the previous section, the formula for a sequence is nothing but the formula for its n^{th} term. Let us see the formulas for n^{th} term (a_{n}) of different types of sequences in math.
 Arithmetic sequence: a_{n} = a + (n  1) d, where a = the first term and d = common difference.
 Geometric sequence: a_{n} = ar^{n1}, where a = the first term and r = common ratio.
 Fibonacci sequence: a_{n+2} = a_{n+1} + a_{n}. The first two terms are 0 and 1.
 Square number sequence: a_{n} = n^{2}.
 Cube number sequence: a_{n} = n^{3}.
 Triangular number sequence: a_{n} = ∑_{k=1}^{n} n. This can be further evaluated using the sum of natural numbers formula.
Finding Missing Numbers
Using the above rules/formulas of sequences, we can find the missing numbers of sequences. Sometimes, we don't need to find the general term also to find the missing terms. If the given sequence doesn't belong to any of the specific sequences mentioned above, then we have to observe the pattern of the sequence and define the general term. Using that we can find the missing numbers.
Example: Find the missing number of the sequence 2, 12, 36, 80, __.
Solution:
It is very clear that the sequence does not belong to any of the sequences that we have mentioned in the previous section. So let us observe the terms. We can see that:
 2 = 1^{2} + 1^{3}
 12 = 2^{2} + 2^{3}
 36 = 3^{2} + 3^{3}
 80 = 4^{2} + 4^{3}
Thus, the missing number would be 5^{2} + 5^{3} = 25 + 125 = 150.
Important Notes on Sequences:
 In an arithmetic sequence, each successive term is obtained by adding the common difference to its preceding term.
 In a geometric sequence, each successive term is obtained by multiplying the common ratio to its preceding term.
 The reciprocal of terms in harmonic sequence form an arithmetic sequence.
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Sequences Examples

Example 1: A taxi charges $2 for the first mile and $1.5 for each subsequent mile. How much does Katie need to pay the taxi driver if she travels 20 miles?
Solution:
The taxi charges for the first few miles are $2, $3.5, $5, ....
This is clearly an arithmetic sequence where the first term is, a = 2 and the common difference is, d = 1.5.
The formula for finding its n^{th} term is,
a_{n} = a + (n  1) d
Substitute n= 20,
a_{20} = 2 + (20  1) (1.5) = $30.5.
Answer: Total charges = $30.5.

Example 2: Flora loves growing flowers in her garden. There is 1 flower in the 1^{st} row, 4 flowers in the 2^{nd} row, 9 flowers in the 3^{rd} row, and so on. In which row did she plant 100 flowers?
Solution:
The sequence of the number of flowers starting from the first row is 1, 4, 9, ....
This is clearly a square number sequence. Its general term is, a_{n} = n^{2}.
Substitute a_{n} = 100 here,
100 = n^{2}
n = 10
Answer: There are 100 flowers in the 10^{t}^{h} row.

Example 3: The 6^{th} term and 11^{th} term of the harmonic sequence are 10 and 18 respectively. Find the common difference between the associated arithmetic sequence.
Solution:
Let 'a' and 'd' be the first term and the common difference of the arithmetic sequence that is associated with the given harmonic sequence.
By the definition of the harmonic sequence,
6^{th} term in the associated arithmetic sequence is, a + 5d = 1/10 ... (1)
11^{th} term in the associated arithmetic sequence is, a + 10d = 1/18 ... (2)
Subtracting (1) from (2),
5d = 2/45
d = 2/225.
Answer: The required common difference is 2/225.
FAQs on Sequences
What is the Definition of Sequence?
A sequence is an ordered list of elements with a specific pattern. For example, 3, 7, 11, 15, ... is a sequence as there is a pattern where each term is obtained by adding 4 to its previous term.
What are 4 Types of Sequences?
There are many types of sequences. The following are the 4 important types of sequences:
 Arithmetic sequence
 Geometric sequence
 Harmonic sequence
 Fibonacci sequence
What is the Formula of Sequence?
The general term (or) n^{th} term defines a sequence. If a sequence belongs to specific types like arithmetic, geometric, etc, then we have formulas to find the general term of the respective sequence. Otherwise, one has to observe the numbers of the sequence to identify the pattern. For example, the sequence 1, 27, 125, ..., does not represent any type of sequence but one can notice that it represents the cubes of odd natural numbers.
What Kind of Sequence is 7, 20, 33, ...?
In the sequence 7, 20, 33, ...,
 second term = 20 = 7 + 13 = first term + 13
 third term = 33 = 20 + 13 = second term + 13
Since every term is obtained by adding 13 to its previous term, it is an arithmetic sequence (which is also known as arithmetic progression).
How to Construct an Arithmetic Sequence?
Follow the steps mentioned below to construct an arithmetic sequence.
 Take any number as the first term.
 Fix a number as a common difference.
 Add the common difference to the first term to obtain the second term.
 To obtain each successive term, keep adding the common difference to its preceding term.
How to Construct a Geometric Sequence?
Follow the steps mentioned below to construct a geometric sequence.
 Take any number as the first term.
 Fix a number as a common ratio.
 Multiply the common ratio to the first term to obtain the second term.
 To obtain each successive term, keep multiplying the common ratio to its preceding term.
What is the Difference Between a Sequence and a Series?
A sequence is a collection of elements that are arranged according to a specific pattern whereas a series is the sum of a few or all elements of the sequence.
How to Find the Sum of Infinite Sequences?
The sum of all infinite sequences may not exist. But we can find the sum of infinite geometric sequence only when its common ratio (r) is less than 1. If its first term is a, then the sum of its infinite terms is a / (1  r).
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