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Sequence and Series
Sequence and series are used in mathematics as well as in our daily lives. A sequence is also known as progression and a series is developed by sequence. Sequence and series is one of the basic concepts in Arithmetic. Sequences are the grouped arrangement of numbers orderly and according to some specific rules, whereas a series is the sum of the elements in the sequence. For example, 2, 4, 6, 8 is a sequence with four elements and the corresponding series will be 2 + 4 + 6+ 8, where the sum of the series or value of the series will be 20.
There are various types of sequences and series depending upon the set of rules that are used to form the sequence and series. Sequence and series are explained in detail below.
What Are Sequence and Series?
The sequence is the group or sequential arrangement of numbers in a particular order or set of rules. Series is formed by adding the terms of a sequence. In a sequence, an individual term can be present in many places. Sequences can be of two types, i.e. infinite sequence and finite sequence and series will be then defined by adding the terms of the sequence. Sum of infinite terms in a series is possible in some cases as well.
Let us understand this with an example. 1, 3, 5, 7, 9, 11, ... is a sequence where there is a common difference of 2 between any two terms and the sequence goes on increasing up to infinity unless the upper limit is given. These types of sequences are known as arithmetic sequences. Now if we add the numbers in the sequence like 1 + 3 + 5 + 7+ 9... this will make a series of this sequence. These kinds of series are known as arithmetic series. A few examples of sequence and series are given in the image shown below:

Difference Between Sequence and Series
The important differences between sequence and series are explained in the table given below:
SequenceSeriesIn sequence, elements are placed in a particular order following a particular set of rules.In series, the order of the elements is not necessary.It is just a collection (set) of elements that follow a pattern.It is a sum of elements that follow a pattern.Order of appearance of the numbers is important.The order of appearance is not important.Example: Harmonic sequence: 1, 1/2, 1/3, 1/4, ...Example: Harmonic series: 1 + 1/2 + 1/3 + 1/4 + ...
Types of Sequence and Series
There are various types of sequences and series, in this section, we will discuss some special and most commonly used sequences and series. The types of sequence and series are:
Arithmetic Sequences and Series
Geometric Sequences and Series
Harmonic Sequences and Series
Arithmetic Sequence and Series
An arithmetic sequence is a sequence where the successive terms are either the addition or subtraction of the common term known as common difference. For example, 1, 4, 7, 10, ...is an arithmetic sequence. A series formed by using an arithmetic sequence is known as the arithmetic series for example 1 + 4 + 7 + 10... is an arithmetic series.
Geometric Sequence and Series
A geometric sequence is a sequence where the successive terms have a common ratio. For example, 1, 4, 16, 64, ...is an arithmetic sequence. A series formed by using geometric sequence is known as the geometric series for example 1 + 4 + 16 + 64... is a geometric series. The geometric progression can be of two types: Finite geometric progression and infinite geometric series.
Harmonic Sequence and Series
A harmonic sequence is a sequence where the sequence is formed by taking the reciprocal of each term of an arithmetic sequence. For example, 1, 1/4, 1/7, 1/10,... is a harmonic sequence. A series formed by using harmonic sequence is known as the harmonic series for example 1 + 1/4 + 1/7 + 1/10.... is a harmonic series.
Sequence and Series Formulas
There are various formulas related to various sequences and series by using them we can find a set of unknown values like the first term, nth term, common parameters, etc. These formulas are different for each kind of sequence and series. Formulas related to various sequences and series are explained below:
Arithmetic sequencea, a + d, a + 2d, a + 3d, ...Arithmetic seriesa + (a + d) + (a + 2d) + (a + 3d) + ...First term:aCommon difference(d):Successive term – Preceding term or an - an-1nth term, ana + (n-1)dSum of arithmetic series, Sn(n/2)(2a + (n-1)d)
Geometric sequencea, ar, ar2,….,ar(n-1),…Geometric seriesa + ar + ar2 + ...+ ar(n-1)+ …First termaCommon ratiornth termar(n-1)Sum of geometric series
Finite series: Sn = a(1−rn)/(1−r) for r≠1, and Sn = an for r = 1
Infinite series: Sn = a/(1−r) for |r| less than 1, and not defined for |r| greater than1