Sequences and Series
Sequences and Series: In mathematics, the sequence is a collection or list of numbers that have a logical/sequential order or pattern between them. For example, 1, 5, 9, 13, β¦ is a sequence having a difference of 4 between each consecutive next term and each term can be represented in form 1 + 4 * ( n β 1 ) where n is the nth term of the sequence. The sequence can be classified into 3 categories:
- Arithmetic Sequence
- Geometric Sequence
- Harmonic Sequence
Table of Content
- What are Sequences and Series?
- Arithmetic Sequence
- Geometric Sequence
- Harmonic Sequence
- What is Summation Notation?
- Summation Notation for Arithmetic Sequence
- Summation Notation for Geometric Sequence
- Summation Notation for Harmonic Sequence
- Sequences and Series Examples
What are Sequences and Series?
A sequence is an ordered list of numbers that follow a specific pattern or rule. Each number in the sequence is called a term. Sequences can be finite or infinite, depending on whether they have a limited number of terms or continue indefinitely. The pattern or rule that defines a sequence can be arithmetic, geometric, or based on some other mathematical relationship.
Arithmetic Sequence
The sequence in which each consecutive term has a common difference and this difference could be positive, negative and even zero is known as an arithmetic sequence.
Example:
1) 0, 2, 4, 6, β¦ in this sequence each and every consecutive term has a difference of 2 between them and nth term of sequence can be represented as 2 * ( n β 1 ).
2) 0, 5, 10, 15, β¦ is another example of arithmetic sequence with a difference of 5 between each consecutive number and nth term of sequence can be represented as 5 * ( n β 1 ).
Geometric Sequence
The sequence in which each consecutive term has a common ratio is known as a Geometric sequence.
Example:
1) 1, 5, 25, 125 β¦ in this sequence, each consecutive term have a ratio of 5 with the term before it and nth term of sequence can be represented as 5 ( n β 1 ).
2) 1, -2, 4, -8, 16, β¦ in this sequence each consecutive term have a ratio of -2 with the term before it and nth term of sequence can be represented as ( -2 )( n β 1 ).
Harmonic Sequence
The sequence in which the reciprocal of each term forms an arithmetic sequence is known as a harmonic sequence.
Example:
(1/5), (1/10), (1/15), (1/20),β¦ in this sequence the reciprocal of each term that is 5, 10, 15, 20, β¦ forms an arithmetic sequence with a difference of 5 between each consecutive term.
What is Summation Notation?
Summation Notation is a simple method to find the sum of a sequence. Summation notation is also known as sigma notation. Sigma refers to the Greek letter sigma, Ξ£. The limit of the sequence is represented as shown in figure 1 where the lower limit is the starting index of the sequence and the upper limit represents the ending index of the sequence. Like as shown in figure 1 the lower limit is 1 and the upper limit is 4 so this means we need the sum of 1st,2nd,3rd and 4th term which is ( 2 * 1 ) + ( 2 * 2 ) + ( 2 * 3 ) + ( 2 * 4 ) = 2 + 4 + 6 + 8 = 20.
Summation Notation:
[Tex]\huge \sum\limits_{n=1}^4 (2*n)[/Tex]
Summation Notation for Arithmetic Sequence
Summation Notation of Arithmetic Sequence is of form Ξ£ (a + b * n) where a is the first term of the sequence and b is the common difference between any two consecutive terms of the sequence and therefore the nth term of the sequence would be of the form (a + (b * n)).
Example:
Let the arithmetic sequence be 0, 2, 4, 6, β¦ so for making the summation notation we need to find the values of βaβ and βbβ where βaβ is the first term which is 0 so a = 0 and b is the common difference between any 2 consecutive terms which is 2 in this case, so b = 2.Therefore summation notion of sequence would be Ξ£ (0 + (2 * n)) where the lower limit is 0 and the upper limit is β as the first term of the sequence is given as 0 and ending is not defined.
Arithmetic Summation Notation:
[Tex]\huge \sum\limits_{n=0}^\infin (0\ +\ 2*n)[/Tex]
Summation Notation for Geometric Sequence
Summation Notation of Geometric Sequence is of form Ξ£ (a * bn) where a is the first term of the sequence and b is the common ratio between any two consecutive terms of the sequence and therefore the nth term of the sequence would be of the form (a * bn).
Example:
Let the geometric sequence be 2, 10, 50, 250, β¦ so for making the summation notation we need to find the values of βaβ and βbβ where βaβ is the first term which is 2 so a = 2 and b is the common ratio between any 2 consecutive terms which is 5 in this case, so b = 5.Therefore summation notion of sequence would be Ξ£ (2 * 5 n) where the lower limit is 0 and the upper limit is β as the first term of the sequence is given as 0 and ending is not defined.
Geometric Summation Notation:
[Tex]\huge \sum\limits_{n=0}^\infin (2\ *\ 5^n)[/Tex]
Summation Notation for Harmonic Sequence
Summation Notation of Harmonic Sequence is of form Ξ£ ( 1/(a + b*n) )where a is the reciprocal of the first term of the sequence and b is the common difference between reciprocal any two consecutive terms of the sequence and therefore the nth term of the sequence would be of the form (1/(a + b * n)).
Example:
Let the arithmetic sequence be 1/2, 1/4, 1/6, β¦ so for making the summation notation we need to find the values of βaβ and βbβ where βaβ is the reciprocal first term which is 2 so a = 2 and b is the common difference between the reciprocal of any 2 consecutive terms which is 2 in this case, so b = 2.Therefore summation notion of sequence would be Ξ£ (1/( 0 + 2 * n)) where the lower limit is 1 and the upper limit is β as the first term of the sequence is given as 1/2 and ending is not defined.
Harmonic Summation Notation:
[Tex]\huge \sum\limits_{n=0}^\infin (\frac{1}{0\ +\ 2^n})[/Tex]
Sequences and Series Examples
Example 1: Find the first 4 terms of the sequence: an = 2 * xn + 1 and n > 0?
Solution:
As we should take care to replace n ( and not x ) with the first 4 natural numbers as n is not equal to 0.
a1 = 2 * x1 + 1
a2 = 2 * x2 + 1
a3 = 2 * x3 + 1
a4 = 2 * x4 + 1
Example 2: Find the first 6 terms of the sequence: an = [Tex]\frac{1}{n\ +\ 2}[/Tex] and n β₯ 0?
Solution:
We have to replace n by the first 6 while numbers (0, 1, 2, 3, 4, 5).
a0 = [Tex]\frac{1}{0\ +\ 2} = \frac{1}{\ 2}[/Tex]
a1 = [Tex]\frac{1}{1\ +\ 2} = \frac{1}{\ 3}[/Tex]
a2 = [Tex]\frac{1}{2\ +\ 2} = \frac{1}{\ 4}[/Tex]
a3 = [Tex]\frac{1}{3\ +\ 2} = \frac{1}{\ 5}[/Tex]
a4 = [Tex]\frac{1}{4\ +\ 2} = \frac{1}{\ 6}[/Tex]
a5 = [Tex]\frac{1}{5\ +\ 2} = \frac{1}{\ 7}[/Tex]
Example 3: Evaluate [Tex]\sum\limits_{n=1}^k (2*n)[/Tex] for k = 2 and k = 4?
Solution:
For k = 2
[Tex]\sum\limits_{n=1}^2 (2*n)[/Tex] = (2 * 1) + (2 * 2) = 2 + 4 = 6
For k = 4
[Tex]\sum\limits_{n=1}^4 (2*n) [/Tex] = (2 * 1) + (2 * 2) + (2 * 3) + (2 * 4) = 2 + 4 + 6 + 8 = 20
Example 4: Evaluate [Tex]\sum\limits_{n=1}^4 (5^n)[/Tex]?
Solution:
[Tex]\sum\limits_{n=1}^4 (5^n)[/Tex] = (51) + (52) + (53) + (54) = 5 + 25 + 125 + 625 = 780
Example 5: Evaluate [Tex]\sum\limits_{n=0}^3 (\frac{1}{n\ +\ 2^n})[/Tex]?
Solution:
[Tex]\sum\limits_{n=0}^2 (\frac{1}{n\ +\ 2^n})= (\frac{1}{0\ +\ 2^0})+(\frac{1}{1\ +\ 2^1})+(\frac{1}{2\ +\ 2^2})\\ \sum\limits_{n=0}^2 (\frac{1}{n\ +\ 2^n})=(\frac{1}{1})+(\frac{1}{3})+(\frac{1}{6})\\ \sum\limits_{n=0}^2 (\frac{1}{n\ +\ 2^n})=(\frac{6+2+1}{6})\\ \sum\limits_{n=0}^2 (\frac{1}{n\ +\ 2^n})=(\frac{9}{6})\\ \sum\limits_{n=0}^2 (\frac{1}{n\ +\ 2^n})=(\frac{3}{2})\\[/Tex]
Example 6: Write in expanded form: [Tex]\sum\limits_{n=0}^\infin (\frac{1}{n\ +\ 2^n})[/Tex] upto 4 terms?
Solution:
[Tex]\sum\limits_{n=0}^\infin (\frac{1}{n\ +\ 2^n}) = (\frac{1}{0\ +\ 2^0}) + (\frac{1}{1\ +\ 2^1})+(\frac{1}{2\ +\ 2^2})+(\frac{1}{3\ +\ 2^3})+β¦\\ \sum\limits_{n=0}^\infin (\frac{1}{n\ +\ 2^n}) = (\frac{1}{1}) + (\frac{1}{3})+(\frac{1}{6})+(\frac{1}{11})+β¦ [/Tex]
Sequences and Series Class 11
Also Check:
Sequences and Series Class 11 Notes
Sequences and Series Class 11 NCERT Solutions
FAQs on Sequences and Series
What is the difference between a sequence and a series?
A sequence is an ordered list of numbers following a specific pattern, while a series is the sum of the terms of a sequence. For example, in the sequence 2, 4, 6, 8, the series would be 2 + 4 + 6 + 8.
How do you find the nth term of an arithmetic sequence?
The nth term of an arithmetic sequence can be found using the formula: an = a1 + (n β 1)d where ana_nanβ is the nth term, a1a_1a1β is the first term, nnn is the term number, and ddd is the common difference.
What is the formula for the sum of the first n terms of a geometric series?
The sum of the first n terms of a geometric series is given by: where Snβ is the sum, a is the first term, r is the common ratio, and n is the number of terms.
How can you determine if a series converges or diverges?
A series converges if the sum of its terms approaches a finite value as more terms are added. For a geometric series, it converges if the absolute value of the common ratio | r | < 1. For other types of series, various tests like the comparison test, ratio test, or integral test are used to determine convergence or divergence.