Sequence and series 

Sequence and series is one of the basic topics in Arithmetic. An itemized collection of elements in which repetitions of any sort are allowed is known as a sequence, whereas series is the sum of all elements. An arithmetic progression is one of the common examples of sequence and series.

Ø  In short, a sequence is a list of items/objects which have been arranged in a sequential way.

Ø  A series can be highly generalized as the sum of all the terms in a sequence. However, there has to be a definite relationship between all the terms of the sequence.

The fundamentals could be better understood by solving problems based on the formulas. They are very similar to sets but the primary difference is that in a sequence, individual terms can occur repeatedly in various positions. The length of a sequence is equal to the number of terms and it can be either finite or infinite.  This concept is explained in a detailed manner in Class 11 Maths. With the help of definition, formulas and examples we are going to discuss here the concepts of sequence as well as series.

Table of contents:

Sequence and Series Definition

A sequence is an arrangement of any objects or a set of numbers in a particular order followed by some rule. If a1, a2, a3, a4,……… etc. denote the terms of a sequence, then 1,2,3,4,…..denotes the position of the term.

A sequence can be defined based on the number of terms i.e. either finite sequence or infinite sequence.

If a1, a2, a3, a4, ……. is a sequence, then the corresponding series is given by

SN = a1+a2+a3 + .. + aN

Note:  The series is finite or infinite depending if the sequence is finite or infinite.

Types of Sequence and Series

Some of the most common examples of sequences are:

       Arithmetic Sequences

       Geometric Sequences

       Harmonic Sequences

       Fibonacci Numbers

Arithmetic Sequences

A sequence in which every term is created by adding or subtracting a definite number to the preceding number is an arithmetic sequence.

Geometric Sequences

A sequence in which every term is obtained by multiplying or dividing a definite number with the preceding number is known as a geometric sequence.

Harmonic Sequences

A series of numbers is said to be in harmonic sequence if the reciprocals of all the elements of the sequence form an arithmetic sequence.

Fibonacci Numbers

Fibonacci numbers form an interesting sequence of numbers in which each element is obtained by adding two preceding elements and the sequence starts with 0 and 1. Sequence is defined as, F0 = 0 and F1 = 1 and Fn = Fn-1 + Fn-2

Also, see:

  • Sequence and Series Worksheets
  • Difference Between Sequence And Series
  • Sequences and Series Class 11
  • Important Questions Class 11 Maths Chapter 9 Sequences Series

Sequence and Series Formulas

List of some basic formula of arithmetic progression and geometric progression are

 

Arithmetic Progression

Geometric Progression

Sequence

a, a+d, a+2d,……,a+(n-1)d,….

a, ar, ar2,….,ar(n-1),…

Common Difference or Ratio

Successive term – Preceding term

Common difference = d = a2 – a1

Successive term/Preceding term

Common ratio = r = ar(n-1)/ar(n-2)

General Term (nth Term)

an = a + (n-1)d

an = ar(n-1)

nth term from the last term

an = l – (n-1)d

an = 1/r(n-1)

Sum of first n terms

sn = n/2(2a + (n-1)d)

sn = a(1 – rn)/(1 – r) if r < 1

sn = a(rn -1)/(r – 1) if r > 1

*Here, a = first term, d = common difference, r = common ratio, n = position of term, l = last term

Difference Between Sequences and Series

Let us find out how a sequence can be differentiated with series.

Sequences

Series

Set of elements that follow a pattern

Sum of elements of the sequence

Order of elements is important

Order of elements is not so important

Finite sequence: 1,2,3,4,5

Finite series: 1+2+3+4+5

Infinite sequence: 1,2,3,4,……

Infinite Series: 1+2+3+4+……

Sequence and Series Examples

Question 1: If 4,7,10,13,16,19,22……is a sequence, Find:

  1. Common difference
  2. nth term
  3. 21st term

Solution: Given sequence is, 4,7,10,13,16,19,22……

a) The common difference = 7 – 4 = 3

b) The nth term of the arithmetic sequence is denoted by the term Tn and is given by Tn = a + (n-1)d, where “a” is the first term and d, is the
common difference.
Tn = 4 + (n – 1)3 = 4 + 3n – 3 = 3n + 1
c) 21st term as:  T21 = 4 + (21-1)3 = 4+60 = 64.

Question 2: Consider the sequence 1, 4, 16, 64, 256, 1024….. Find the common ratio and 9th term.

Solution: The common ratio (r)  = 4/1 = 4

The preceding term is multiplied by 4 to obtain the next term.

The nth term of the geometric sequence is denoted by the term Tn and is given by Tn = ar(n-1)
where a is the first term and r is the common ratio.

Here a = 1, r = 4 and n = 9

So, 9th term is can be calculated as T9 = 1* (4)(9-1)= 48 = 65536.

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