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# NCERT Solutions of Class 11 Maths Exercise 9.1 – Sequences and Series

NCERT Solutions of class 11 maths exercise 9.1 of chapter 9-‘Sequences and series’ are the solutions of unsolved question exercise 9.1 of chapter 9 of class 9 NCERT textbook prescribed by CBSE  a reputed board for school education.All questions of exercise 9.1 of chapter 9 of the maths NCERT textbook are solved by an expert of maths by a step by step method, so every student can understand the solutions quickly.

In class 10 you have studied one part of Sequences and Series, the chapter ‘Arithmetic Progression’, now in class 11 you will study it in more advance with other important series geometrical progression and other complex series.

## NCERT Solutions For Class 11 Maths  of Chapter 9 Sequences and Series

Exercise 9.2 – Sequence and Series

Exercise 9.3 – Sequence and Series

Exercise 9.4 – Sequence and Series

Study notes of Maths and Science NCERT and CBSE from class 9 to 12

## NCERT Solutions of exercise 9.1 ‘ Sequences and Series

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Q1.Write the first 5 terms of the sequence whose n the term is an = n(n +2).

Ans. The n the term of the series is given to us = n(n + 2)

The first term(n=1) of the series = 1(1 +2) = 3

Second term(n =2) of the series = 2(2 + 2) = 8

Third term of the series(n =3) = 3(3 + 2) = 15

Fourth term of the series (n =4) = 4( 4 +2) =24

Fifth term of the series (n =5) = 5(5 + 2) = 35

Q2.Write the first 5 terms of the sequences whose n the term is

Ans. The n the term given to us is

Substituting n by the first 5 numbers 1,2,3,4 and 5, we will get first 5 terms of the sequences.

Hence the first 5 terms of the sequences are 1/2,2/3,3/4, 4/5 and 5/6

Q3. Write the first five terms of the sequences whose n the term is an = 2n

Ans. We are given the n the term

an = 2n

Substituting n by first numbers, we will get first 5 terms

a1 = 21= 2, a2 = 22 = 4, 23 = 8, 24 =16, 25 = 32

Hence first 5 terms of the sequences are 2,4,8,16 and 32.

Q4.Write the first five terms of the sequences whose n the term is

Ans. We are given the n the term

Substituting n by first 5 numbers, 1,2,3,4 and 5 we will get first 5 terms

Hence first 5 terms of the sequences are -1/6,1/6, 1/2,5/6 and 7/6

Q5. .Write the first five terms of the sequences whose n the term is an = (-1)n-15n+1

Ans.We are given the n the term, an = (-1)n-15n+1

Substituting n by first 5 numbers, 1,2,3,4 and 5 we will get first 5 terms

a1 = (-1)1-151+1,=(-1)05² = 5² = 25

a2 = (-1)2-152+1,=(-1)15³ = -1×5³ = -125

a3 = (-1)3-153+1 =(-1)254 = 625

a4 = (-1)4-154+1 =(-1)355 = -3125

a5= (-1)5-155+1 =(-1)456 = -15625

Hence first 5 terms of the sequences are

Q5. .Write the first five terms of the sequences whose n the term is

Ans.We are given the n the term

Substituting n by first 5 numbers, 1,2,3,4 and 5 we will get first 5 terms

Hence first 5 terms of the sequences are 3/2, 9/2, 21/2, 21 and 75/2

Q7. Find 17 the and 24 term in the following sequence whose n th term is an = 4n -3

Ans.  We are given the n th term, an = 4n -3

Substituting n = 17 and n =24 in the n th term of the sequences

a17 = 4×17 -3=68-3=65

a24 = 4×24 -3=96-3=93

Q8.Find the seventh term in the following sequence whose n th term is

Ans.We are given the n th term of the sequence

Substituting n = 7

Q9..Find the nine th term in the following sequence whose n th term is

an = (-1)n-1n3

Ans. We are given the n th term of the sequence

an = (-1)n-1n3

Substituting n = 9

a9= (-1)9-193,=1X729 = 729

Q10.Find the 20 th term in the following sequence whose n th term is

Ans. We are given the n th term of the sequence

Substituting n = 20

Q11.Write the first 5 terms of the following sequence and obtain the corresponding series.

an = 3an-1 +2 for all n >2.

Ans. We are given  a1=3  and an = 3an-1 +2 for all n >2

The n th term of the sequence is

an = 3an-1 +2 for all n >2

Substituting n = 2,3,4,5 we have

a2 = 3a2-1 +2 = 3a1 +2=3×3+2=11

a3 = 3a3-1 +2 = 3a2+2=3×11+2=35

a4 = 3a4-1 +2 = 3a3+2=3×35+2=107

a5 = 3a5-1 +2 = 3a4+2=3×107+2=323

First 5 terms of the sequence are 3,11,35,107 and 323

Try question number from 12 to 14

Q12.Write the first 5 terms of the following sequence and obtain the corresponding series.

Ans. We are given the first and nth term of the sequence

Substituting n = 2,3,4,5 we have

The second term of the sequence

Therefore first 5 terms of the series are -1,-1/2,-1/6,-1/24 and -1/120

Q13Write the first 5 terms of the following sequence and obtain the corresponding series.

a1 = a2=2, an = an-1 – 1, n≥ 2

Ans. Given :a1 = a2=2, an = an-1 – 1

Then , a3 = a2 – 1 = 2 – 1 = 1

a4 = a3 – 1 = 1 – 1 = 0

a5 = a4 – 1 = 0 – 1 = -1

Thus , the first five terms of the sequence are 2 ,2 ,1 ,0 and -1

The corresponding series is

2 + 2 + 1 + 0 +(- 1)+…

Q14.The Fibonacci sequence is defined by

Ans.Given,

a3 = a2 + a1 =  1 + 1  = 2

a4 = a3 + a2  = 2 + 1  = 3

a5 = a4 + a3  = 3 + 2 = 5

Thus ,

For n = 1,2,3,4 and 5 the sequence is given as following

For n =1

For n =2

For n=3

For n= 4

For n= 5

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