NCERT Solutions for Class 11 Maths Chapter 9 - Sequences and Series

*According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 8.

Practising the NCERT Solutions Class 11 Chapter 9 Sequences and Series can help the students develop a thorough understanding of the topics explained in the Chapter. These solutions are prepared by highly experienced teachers at BYJU’S according to the latest CBSE Syllabus 2023-24 to help the students in attempting challenging questions with utmost confidence. Using the NCERT Solutions provided here, students can learn new methods of solving a particular problem in an expeditious time to improve their performance in the Class 11 Maths exam. These solutions have been designed after undertaking extensive research on each question and their problem-solving methods.

NCERT Solutions for Class 11 Maths Chapter 9 – Sequences and Series

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Access exercise-wise NCERT Solutions for Class 11 Maths Chapter 9 from the below links

Exercise 9.1 Solutions: 14 Questions

Exercise 9.2 Solutions: 18 Questions

Exercise 9.3 Solutions: 32 Questions

Exercise 9.4 Solutions: 10 Questions

Miscellaneous Exercise on Chapter 9 Solutions: 32 Questions

Access Answers to NCERT Class 11 Maths Chapter 9 – Sequences and Series

Exercise 9.1 Page No: 180

Write the first five terms of each of the sequences in Exercises 1 to 6 whose nth terms are:

1. a_n = n (n + 2) 

Solution:

Given,

n^th term of a sequence a_n = n (n + 2) 

On substituting n = 1, 2, 3, 4, and 5, we get the first five terms

a₁ = 1(1 + 2) = 3

a₂ = 2(2 + 2) = 8

a₃ = 3(3 + 2) = 15

a₄ = 4(4 + 2) = 24

a₅ = 5(5 + 2) = 35

Hence, the required terms are 3, 8, 15, 24, and 35.

2. a_n = n/n+1

Solution:

Given the n^th term, a_n = n/n+1

On substituting n = 1, 2, 3, 4, 5, we get

Hence, the required terms are 1/2, 2/3, 3/4, 4/5 and 5/6.

3. a_n = 2ⁿ

Solution:

Given the n^th term, a_n = 2ⁿ

On substituting n = 1, 2, 3, 4, 5, we get

a₁ = 2¹ = 2

a₂ = 2² = 4

a₃ = 2³ = 8

a₄ = 2⁴ = 16

a₅ = 2⁵ = 32

Hence, the required terms are 2, 4, 8, 16, and 32.

4.  a_n = (2n – 3)/6

Solution:

Given the n^th term, a_n = (2n – 3)/6

On substituting n = 1, 2, 3, 4, 5, we get

Hence, the required terms are -1/6, 1/6, 1/2, 5/6 and 7/6..

5. a_n = (-1)^n-1 5ⁿ⁺¹

Solution:

Given the n^th term, a_n = (-1)^n-1 5ⁿ⁺¹

On substituting n = 1, 2, 3, 4, 5, we get

Hence, the required terms are 25, –125, 625, –3125, and 15625.

6.

Solution:

On substituting n = 1, 2, 3, 4, 5, we get the first 5 terms.

Hence, the required terms are 3/2, 9/2, 21/2, 21 and 75/2.

Find the indicated terms in each of the sequences in Exercises 7 to 10 whose n^th terms are:

7. a_n = 4n – 3; a₁₇, a₂₄

Solution:

Given,

The n^th term of the sequence is a_n = 4n – 3

On substituting n = 17, we get

a₁₇ = 4(17) – 3 = 68 – 3 = 65

Next, on substituting n = 24, we get

a₂₄ = 4(24) – 3 = 96 – 3 = 93

8. a_n = n²/2ⁿ ; a⁷

Solution:

Given,

The n^th term of the sequence is a_n = n²/2ⁿ

Now, on substituting n = 7, we get

a₇ = 7²/2⁷ = 49/ 128

9. a_n = (-1)^n-1 n³; a₉

Solution:

Given,

The n^th term of the sequence is a_n = (-1)^n-1 n³

On substituting n = 9, we get

a₉ = (-1)^9-1 (9)³ = 1 x 729 = 729

10.

Solution:

On substituting n = 20, we get

Write the first five terms of each of the sequences in Exercises 11 to 13 and obtain the corresponding series:

11. a₁ = 3, a_n = 3a_n-1 + 2 for all n > 1

Solution:

Given, a_n = 3a_n-1 + 2 and a₁ = 3

Then,

a₂ = 3a₁ + 2 = 3(3) + 2 = 11

a₃ = 3a₂ + 2 = 3(11) + 2 = 35

a₄ = 3a₃ + 2 = 3(35) + 2 = 107

a₅ = 3a₄ + 2 = 3(107) + 2 = 323

Thus, the first 5 terms of the sequence are 3, 11, 35, 107 and 323.

Hence, the corresponding series is

3 + 11 + 35 + 107 + 323 …….

12. a₁ = -1, a_n = a_n-1/n, n ≥ 2

Solution:

Given,

a_n = a_n-1/n and a₁ = -1

Then,

a₂ = a₁/2 = -1/2

a₃ = a₂/3 = -1/6

a₄ = a₃/4 = -1/24

a₅ = a₄/5 = -1/120

Thus, the first 5 terms of the sequence are -1, -1/2, -1/6, -1/24 and -1/120.

Hence, the corresponding series is

-1 + (-1/2) + (-1/6) + (-1/24) + (-1/120) + …….

13. a₁ = a₂ = 2, a_n = a_n-1 – 1, n > 2

Solution:

Given,

a₁ = a₂, a_n = a_n-1 – 1

Then,

a₃ = a₂ – 1 = 2 – 1 = 1

a₄ = a₃ – 1 = 1 – 1 = 0

a₅ = a₄ – 1 = 0 – 1 = -1

Thus, the first 5 terms of the sequence are 2, 2, 1, 0 and -1.

The corresponding series is

2 + 2 + 1 + 0 + (-1) + ……

14. The Fibonacci sequence is defined by

1 = a₁ = a₂ and a_n = a_{n – 1} + a_{n – 2}, n > 2

Find a_n+1/a_n, for n = 1, 2, 3, 4, 5 

Solution:

Given,

1 = a₁ = a₂

a_n = a_{n – 1} + a_{n – 2}, n > 2

So,

a₃ = a₂ + a₁ = 1 + 1 = 2

a₄ = a₃ + a₂ = 2 + 1 = 3

a₅ = a₄ + a₃ = 3 + 2 = 5

a₆ = a₅ + a₄ = 5 + 3 = 8

Thus,

Exercise 9.2 Page No: 185

1. Find the sum of odd integers from 1 to 2001.

Solution:

The odd integers from 1 to 2001 are 1, 3, 5, …1999, 2001.

It clearly forms a sequence in A.P.

Where the first term, a = 1

The common difference, d = 2

Now,

a + (n -1)d = 2001

1 + (n-1)(2) = 2001

2n – 2 = 2000

2n = 2000 + 2 = 2002

n = 1001

We know,

S_n = n/2 [2a + (n-1)d]

Therefore, the sum of odd numbers from 1 to 2001 is 1002001.

2. Find the sum of all natural numbers lying between 100 and 1000, which are multiples of 5.

Solution:

The natural numbers lying between 100 and 1000, which are multiples of 5, are 105, 110, … 995.

It clearly forms a sequence in A.P.

Where the first term, a = 105

The common difference, d = 5

Now,

a + (n -1)d = 995

105 + (n – 1)(5) = 995

105 + 5n – 5 = 995

5n = 995 – 105 + 5 = 895

n = 895/5

n = 179

We know,

S_n = n/2 [2a + (n-1)d]

Therefore, the sum of all natural numbers lying between 100 and 1000, which are multiples of 5, is 98450.

3. In an A.P, the first term is 2, and the sum of the first five terms is one-fourth of the next five terms. Show that the 20^th term is –112.

Solution:

Given,

The first term (a) of an A.P = 2

Let’s assume d is the common difference of the A.P.

So, the A.P. will be 2, 2 + d, 2 + 2d, 2 + 3d, …

Then,

Sum of first five terms = 10 + 10d

Sum of next five terms = 10 + 35d

From the question, we have

10 + 10d = ¼ (10 + 35d)

40 + 40d = 10 + 35d

30 = -5d

d = -6

a₂₀ = a + (20 – 1)d = 2 + (19) (-6) = 2 – 114 = -112

Therefore, the 20^th term of the A.P. is –112.

4. How many terms of the A.P. -6, -11/2, -5, …. are needed to give the sum –25?

Solution:

Let’s consider the sum of n terms of the given A.P. as –25.

We known that,

S_n = n/2 [2a + (n-1)d]

where n = number of terms, a = first term, and d = common difference

So here, a = –6

d = -11/2 + 6 = (-11 + 12)/ 2 = 1/2

Thus, we have

5. In an A.P., if p^th term is 1/q and q^th term is 1/p, prove that the sum of first pq terms is ½ (pq + 1) where p ≠ q.  

Solution:

6. If the sum of a certain number of terms of the A.P. 25, 22, 19, … is 116. Find the last term.

Solution:

Given A.P.,

25, 22, 19, …

Here,

First term, a = 25 and

Common difference, d = 22 – 25 = -3

Also given, the sum of a certain number of terms of the A.P. is 116.

The number of terms is n.

So, we have

S_n = n/2 [2a + (n-1)d] = 116

116 = n/2 [2(25) + (n-1)(-3)]

116 x 2 = n [50 – 3n + 3]

232 = n [53 – 3n]

232 = 53n – 3n²

3n² – 53n + 232 = 0

3n² – 24n – 29n+ 232 = 0

3n(n – 8) – 29(n – 8) = 0

(3n – 29) (n – 8) = 0

Hence,

n = 29/3 or n = 8

As n can only be an integral value, n = 8

Thus, the 8^th term is the last term of the A.P.

a₈ = 25 + (8 – 1)(-3)

= 25 – 21

= 4

7. Find the sum to n terms of the A.P., whose k^th term is 5k + 1.

Solution:

Given, the k^th term of the A.P. is 5k + 1.

k^th term = a_k = a + (k – 1)d

And,

a + (k – 1)d = 5k + 1

a + kd – d = 5k + 1

On comparing the coefficient of k, we get d = 5

a – d = 1

a – 5 = 1

⇒ a = 6

8. If the sum of n terms of an A.P. is (pn + qn²), where p and q are constants, find the common difference.

Solution:

We know that,

S_n = n/2 [2a + (n-1)d]

From the question, we have

On comparing the coefficients of n² on both sides, we get

d/2 = q

Hence, d = 2q

Therefore, the common difference of the A.P. is 2q.

9. The sums of n terms of two arithmetic progressions are in the ratio 5n + 4: 9n + 6. Find the ratio of their 18^th terms.

Solution:

Let a₁, a₂, and d₁, d₂ be the first terms and the common difference of the first and second arithmetic progression, respectively.

Then, from the question, we have

10. If the sum of the first p terms of an A.P. is equal to the sum of the first q terms, then find the sum of the first (p + q) terms.

Solution:

Let’s take a and d to be the first term and the common difference of the A.P., respectively.

Then, it is given that

Therefore, the sum of (p + q) terms of the A.P. is 0.

11. Sum of the first p, q and r terms of an A.P. are a, b and c, respectively.

Prove that 

Solution:

Let a₁ and d be the first term and the common difference of the A.P., respectively.

Then, according to the question, we have

Now, subtracting (2) from (1), we get

12. The ratio of the sums of m and n terms of an A.P. is m²: n². Show that the ratio of the m^th and the n^th term is (2m – 1): (2n – 1).

Solution:

Let’s consider that a and b are the first term and the common difference of the A.P., respectively.

Then, from the question, we have

Hence, the given result is proved.

13. If the sum of n terms of an A.P. is 3n² + 5n and its m^th term is 164, find the value of m.

Solution:

Let’s consider a and b to be the first term and the common difference of the A.P., respectively.

a_m = a + (m – 1)d = 164 … (1)

The sum of the terms is given by,

S_n = n/2 [2a + (n-1)d]

14. Insert five numbers between 8 and 26 such that the resulting sequence is an A.P.

Solution:

Let’s assume A₁, A₂, A₃, A₄, and A₅ to be five numbers between 8 and 26 such that 8, A₁, A₂, A₃, A₄, A₅, 26 are in an A.P.

Here, we have,

a = 8, b = 26, n = 7

So,

26 = 8 + (7 – 1) d

6d = 26 – 8 = 18

d = 3

Now,

A₁ = a + d = 8 + 3 = 11

A₂ = a + 2d = 8 + 2 × 3 = 8 + 6 = 14

A₃ = a + 3d = 8 + 3 × 3 = 8 + 9 = 17

A₄ = a + 4d = 8 + 4 × 3 = 8 + 12 = 20

A₅ = a + 5d = 8 + 5 × 3 = 8 + 15 = 23

Therefore, the required five numbers between 8 and 26 are 11, 14, 17, 20, and 23.

15. If is the A.M. between a and b, then find the value of n.

Solution:

The A.M between a and b is given by (a + b)/2

Then, according to the question,

Thus, the value of n is 1.

16. Between 1 and 31, m numbers have been inserted in such a way that the resulting sequence is an A.P. and the ratio of 7^th and (m – 1)^th numbers is 5: 9. Find the value of m.

Solution:

Let’s consider a₁, a₂, … a_m be m numbers such that 1, a₁, a₂, … a_m, 31 is an A.P.

And here,

a = 1, b = 31, n = m + 2

So, 31 = 1 + (m + 2 – 1) (d)

30 = (m + 1) d

d = 30/ (m + 1) ……. (1)

Now,

a₁ = a + d

a₂ = a + 2d

a₃ = a + 3d …

Hence, a₇ = a + 7d

a_m–1 = a + (m – 1) d

According to the question, we have

Therefore, the value of m is 14.

17. A man starts repaying a loan as the first instalment of Rs. 100. If he increases the instalment by Rs 5 every month, what amount will he pay in the 30^th instalment?

Solution:

Given,

The first instalment of the loan is Rs 100.

The second instalment of the loan is Rs 105, and so on as the instalment increases by Rs 5 every month.

Thus, the amount that the man repays every month forms an A.P.

And then, A.P. is 100, 105, 110, …

Where the first term, a = 100

Common difference, d = 5

So, the 30^th term in this A.P. will be

A₃₀  = a + (30 – 1)d

= 100 + (29) (5)

= 100 + 145

= 245

Therefore, the amount to be paid in the 30^th instalment will be Rs 245.

18. The difference between any two consecutive interior angles of a polygon is 5°. If the smallest angle is 120°, find the number of the sides of the polygon.

Solution:

It’s understood from the question that the angles of the polygon will form an A.P. with a common difference d = 5° and first term a = 120°.

And we know that the sum of all angles of a polygon with n sides is 180° (n – 2).

Thus, we can say

Thus, a polygon having 9 and 16 sides will satisfy the condition in the question.

Exercise 9.3 Page No: 192

1. Find the 20^th and n^th terms of the G.P. 5/2, 5/4, 5/8, ………

Solution:

Given G.P. is 5/2, 5/4, 5/8, ………

Here, a = First term = 5/2

r = Common ratio = (5/4)/(5/2) = ½

Thus, the 20^th term and n^th term

2. Find the 12^th term of a G.P. whose 8^th term is 192, and the common ratio is 2.

Solution:

Given,

The common ratio of the G.P., r = 2

And, let a be the first term of the G.P.

Now,

a₈ = ar ^8–1 = ar⁷

ar⁷ = 192

a(2)⁷ = 192

a(2)⁷ = (2)⁶ (3)

3. The 5^th, 8^th and 11^th terms of a G.P. are p, q and s, respectively. Show that q² = ps.

Solution:

Let’s take a to be the first term and r to be the common ratio of the G.P.

Then, according to the question, we have

a₅ = a r^5–1 = a r⁴ = p … (i)

a₈ = a r^8–1 = a r⁷ = q … (ii)

a₁₁ = a r^11–1 = a r¹⁰ = s … (iii)

Dividing equation (ii) by (i), we get

4. The 4^th term of a G.P. is the square of its second term, and the first term is –3. Determine its 7^th term.

Solution:

Let’s consider a to be the first term and r to be the common ratio of the G.P.

Given, a = –3

And we know that,

a_n = ar^n–1

So, a₄ = ar³ = (–3) r³

a₂ = a r¹ = (–3) r

Then, from the question, we have

(–3) r³ = [(–3) r]²

⇒ –3r³ = 9 r²

⇒ r = –3

a₇ = a r ^7–1 = a r⁶ = (–3) (–3)⁶ = – (3)⁷ = –2187

Therefore, the seventh term of the G.P. is –2187.

5. Which term of the following sequences:

(a) 2, 2√2, 4,… is 128 ? (b) √3, 3, 3√3,… is 729 ?

(c) 1/3, 1/9, 1/27, … is 1/19683 ?

Solution:

(a) The given sequence, 2, 2√2, 4,…

We have,

a = 2 and r = 2√2/2 = √2

Taking the n^th term of this sequence as 128, we have

Therefore, the 13^th term of the given sequence is 128.

(ii) Given the sequence, √3, 3, 3√3,…

We have,

a = √3 and r = 3/√3 = √3

Taking the n^th term of this sequence to be 729, we have

Therefore, the 12^th term of the given sequence is 729.

(iii) Given sequence, 1/3, 1/9, 1/27, …

a = 1/3 and r = (1/9)/(1/3) = 1/3

Taking the n^th term of this sequence to be 1/19683, we have

Therefore, the 9^th term of the given sequence is 1/19683.

6. For what values of x, the numbers -2/7, x, -7/2 are in G.P?

Solution:

The given numbers are -2/7, x, -7/2

Common ratio = x/(-2/7) = -7x/2

Also, common ratio = (-7/2)/x = -7/2x

Therefore, for x = ± 1, the given numbers will be in G.P.

7. Find the sum to 20 terms in the geometric progression 0.15, 0.015, 0.0015 …

Solution:

Given G.P., 0.15, 0.015, 0.00015, …

Here, a = 0.15 and r = 0.015/0.15 = 0.1 

8. Find the sum to n terms in the geometric progression √7, √21, 3√7, ….

Solution:

The given G.P. is √7, √21, 3√7, ….

Here,

a = √7 and

9. Find the sum to n terms in the geometric progression 1, -a, a², -a³ …. (if a ≠ -1)

Solution:

The given G.P. is 1, -a, a², -a³ ….

Here, the first term = a₁ = 1

And the common ratio = r = – a

We know that,

10. Find the sum to n terms in the geometric progression x³, x⁵, x⁷, … (if x ≠ ±1 )

Solution:

Given G.P. is x³, x⁵, x⁷, …

Here, we have a = x³ and r = x⁵/x³ = x²

11. Evaluate:

Solution:

12. The sum of the first three terms of a G.P. is 39/10, and their product is 1. Find the common ratio and the terms.

Solution:

Let a/r, a, ar be the first three terms of the G.P.

a/r + a + ar = 39/10 …… (1)

(a/r) (a) (ar) = 1 …….. (2)

From (2), we have

a³ = 1

Hence, a = 1 [Considering real roots only]

Substituting the value of a in (1), we get

1/r + 1 + r = 39/10

(1 + r + r²)/r = 39/10

10 + 10r + 10r² = 39r

10r² – 29r + 10 = 0

10r² – 25r – 4r + 10 = 0

5r(2r – 5) – 2(2r – 5) = 0

(5r – 2) (2r – 5) = 0

Thus,

r = 2/5 or 5/2

Therefore, the three terms of the G.P. are 5/2, 1 and 2/5.

13. How many terms of G.P. 3, 3², 3³, … are needed to give the sum 120?

Solution:

Given G.P. is 3, 3², 3³, …

Let’s consider that n terms of this G.P. be required to obtain the sum 120.

We know that,

Here, a = 3 and r = 3

Equating the exponents, we get n = 4

Therefore, four terms of the given G.P. are required to obtain the sum 120.

14. The sum of the first three terms of a G.P. is 16, and the sum of the next three terms is 128. Determine the first term, the common ratio and the sum to n terms of the G.P.

Solution:

Let’s assume the G.P. to be a, ar, ar², ar³, …

Then, according to the question, we have

a + ar + ar² = 16 and ar³ + ar⁴ + ar⁵ = 128

a (1 + r + r²) = 16 … (1) and,

ar³(1 + r + r²) = 128 … (2)

Dividing equation (2) by (1), we get

r³ = 8

r = 2

Now, using r = 2 in (1), we get

a (1 + 2 + 4) = 16

a (7) = 16

a = 16/7

Now, the sum of terms is given as

15. Given a G.P. with a = 729 and 7^th term 64, determine S₇.

Solution:

Given,

a = 729 and a₇ = 64

Let r be the common ratio of the G.P.

Then, we know that, a_n = a r^n–1

a₇ = ar^7–1 = (729)r⁶

⇒ 64 = 729 r⁶

r⁶ = 64/729

r⁶ = (2/3)⁶

r = 2/3

And we know that

16. Find a G.P. for which the sum of the first two terms is –4 and the fifth term is 4 times the third term.

Solution:

Consider a to be the first term and r to be the common ratio of the G.P.

Given, S₂ = -4

Then, from the question, we have

And,

a₅ = 4 x a₃

ar⁴ = 4ar²

r² = 4

r = ± 2

Using the value of r in (1), we have

Therefore, the required G.P is

-4/3, -8/3, -16/3, …. Or 4, -8, 16, -32, ……

17. If the 4^th, 10^th and 16^th terms of a G.P. are x, y and z, respectively. Prove that x, y, and z are in G.P.

Solution:

Let a be the first term and r be the common ratio of the G.P.

According to the given condition,

a₄ = a r³ = x … (1)

a₁₀ = a r⁹ = y … (2)

a₁₆ = a r¹⁵ = z … (3)

On dividing (2) by (1), we get

18. Find the sum to n terms of the sequence, 8, 88, 888, 8888…

Solution:

Given sequence: 8, 88, 888, 8888…

This sequence is not a G.P.

But, it can be changed to G.P. by writing the terms as

S_n = 8 + 88 + 888 + 8888 + …………….. to n terms

19. Find the sum of the products of the corresponding terms of the sequences 2, 4, 8, 16, 32 and 128, 32, 8, 2, 1/2.

Solution:

The required sum = 2 x 128 + 4 x 32 + 8 x 8 + 16 x 2 + 32 x ½

= 64[4 + 2 + 1 + ½ + 1/2²]

Now, it’s seen that

4, 2, 1, ½, 1/2² is a G.P.

With the first term, a = 4

Common ratio, r =1/2

We know,

Therefore, the required sum = 64(31/4) = (16)(31) = 496

20. Show that the products of the corresponding terms of the sequences a, ar, ar², …ar^n-1 and A, AR, AR², … AR^n-1 form a G.P, and find the common ratio.

Solution:

To be proved: The sequence, aA, arAR, ar²AR², …ar^n–1AR^n–1, forms a G.P.

Now, we have

Therefore, the above sequence forms a G.P., and the common ratio is rR.

21. Find four numbers forming a geometric progression in which the third term is greater than the first term by 9, and the second term is greater than the 4^th by 18.

Solution:

Consider a to be the first term and r to be the common ratio of the G.P.

Then,

a₁ = a, a₂ = ar, a₃ = ar², a₄ = ar³

From the question, we have

a₃ = a₁ + 9

ar² = a + 9 … (i)

a₂ = a₄ + 18

ar = ar³ + 18 … (ii)

So, from (1) and (2), we get

a(r² – 1) = 9 … (iii)

ar (1– r²) = 18 … (iv)

Now, dividing (4) by (3), we get

-r = 2

r = -2

On substituting the value of r in (i), we get

4a = a + 9

3a = 9

∴ a = 3

Therefore, the first four numbers of the G.P. are 3, 3(– 2), 3(–2)², and 3(–2)³

i.e., 3¸–6, 12, and –24.

22. If the p^th, q^th and r^th terms of a G.P. are a, b and c, respectively. Prove that a^q-r b^r-p c^p-q = 1

Solution:

Let’s take A to be the first term and R to be the common ratio of the G.P.

Then, according to the question, we have

AR^p–1 = a

AR^q–1 = b

AR^r–1 = c

Then,

a^q–r b^r–p c^p–q

= A^q–r × R^{(p–1) (q–r)} × A^r–p × R^{(q–1) (r–p)} × A^p–q × R^{(r –1)(p–q)}

= Aq ^{– r + r – p + p – q} × R ^{(pr – pr – q + r) + (rq – r + p – pq) + (pr – p – qr + q)}

= A⁰ × R⁰

= 1

Hence proved.

23. If the first and the n^th term of a G.P. are a and b, respectively, and if P is the product of n terms, prove that P² = (ab)ⁿ.

Solution:

Given the first term of the G.P is a, and the last term is b.

Thus,

The G.P. is a, ar, ar², ar³, … ar^n–1, where r is the common ratio.

Then,

b = ar^n–1  … (1)

P = Product of n terms

= (a) (ar) (ar²) … (ar^n–1)

= (a × a ×…a) (r × r² × …r^n–1)

= aⁿ r ^{1 + 2 +…(n–1)}  … (2)

Here, 1, 2, …(n – 1) is an A.P.

And, the product of n terms P is given by,

24. Show that the ratio of the sum of the first n terms of a G.P. to the sum of terms from .

Solution:

Let a be the first term and r be the common ratio of the G.P.

Since there are n terms from (n +1)^th to (2n)^th term,

Sum of terms from(n + 1)^th to (2n)^th term

a ^n +1 = ar ^{n + 1 – 1} = arⁿ

Thus, the required ratio =

Thus, the ratio of the sum of the first n terms of a G.P. to the sum of terms from (n + 1)^th to (2n)^th term is ^.

25. If a, b, c and d are in G.P., show that (a² + b² + c²)(b² + c² + d²) = (ab + bc + cd)².

Solution:

Given a, b, c, d are in G.P.

So, we have

bc = ad  … (1)

b² = ac  … (2)

c² = bd  … (3)

Taking the R.H.S., we have

R.H.S.

= (ab + bc + cd)²

= (ab + ad + cd)²  [Using (1)]

= [ab + d (a + c)]²

= a²b² + 2abd (a + c) + d² (a + c)²

= a²b² +2a²bd + 2acbd + d²(a² + 2ac + c²)

= a²b² + 2a²c² + 2b²c² + d²a² + 2d²b² + d²c²  [Using (1) and (2)]

= a²b² + a²c² + a²c² + b²c² + b²c² + d²a² + d²b² + d²b² + d²c²

= a²b² + a²c² + a²d² + b² × b² + b²c² + b²d² + c²b² + c² × c² + c²d²

= a²(b² + c² + d²) + b² (b² + c² + d²) + c² (b²+ c² + d²)

= (a² + b² + c²) (b² + c² + d²)

= L.H.S.

Thus, L.H.S. = R.H.S.

Therefore,

(a² + b² + c²)(b² + c² + d²) = (ab + bc + cd)²

26. Insert two numbers between 3 and 81 so that the resulting sequence is G.P.

Solution:

Let’s assume G₁ and G₂ to be two numbers between 3 and 81 such that the series 3, G₁, G₂, 81 forms a G.P.

And let a be the first term and r be the common ratio of the G.P.

Now, we have the 1^st term as 3 and the 4^th term as 81.

81 = (3) (r)³

r³ = 27

∴ r = 3 (Taking real roots only)

For r = 3,

G₁ = ar = (3) (3) = 9

G₂ = ar² = (3) (3)² = 27

Therefore, the two numbers which can be inserted between 3 and 81 so that the resulting sequence becomes a G.P are 9 and 27.

27. Find the value of n so that may be the geometric mean between a and b.

Solution:

We know that,

The G. M. of a and b is given by √ab.

Then from the question, we have

By squaring both sides, we get

28. The sum of two numbers is 6 times their geometric mean; show that numbers are in the ratio.

Solution:

Consider the two numbers to be a and b.

Then, G.M. = √ab.

From the question, we have

29. If A and G be A.M. and G.M., respectively, between two positive numbers, prove that the

numbers are.

Solution:

Given that A and G are A.M. and G.M. between two positive numbers.

And, let these two positive numbers be a and b.

30. The number of bacteria in a certain culture doubles every hour. If there were 30 bacteria present in the culture originally, how many bacteria will be present at the end of the 2^nd hour, 4^th hour and n^th hour?

Solution:

Given the number of bacteria doubles every hour. Hence, the number of bacteria after every hour will form a G.P.

Here we have, a = 30 and r = 2

So, a₃ = ar² = (30) (2)² = 120

Thus, the number of bacteria at the end of 2^nd hour will be 120.

And, a₅ = ar⁴ = (30) (2)⁴ = 480

The number of bacteria at the end of 4^th hour will be 480.

a_n +1 = arⁿ = (30) 2ⁿ

Therefore, the number of bacteria at the end of n^th hour will be 30(2)ⁿ.

31. What will Rs 500 amount to in 10 years after its deposit in a bank which pays an annual interest rate of 10% compounded annually?

Solution:

Given,

The amount deposited in the bank is Rs 500.

At the end of first year, amount = Rs 500(1 + 1/10) = Rs 500 (1.1)

At the end of 2^nd year, amount = Rs 500 (1.1) (1.1)

At the end of 3^rd year, amount = Rs 500 (1.1) (1.1) (1.1) and so on….

Therefore,

The amount at the end of 10 years = Rs 500 (1.1) (1.1) … (10 times)

= Rs 500(1.1)¹⁰

32. If A.M. and G.M. of roots of a quadratic equation are 8 and 5, respectively, then obtain the quadratic equation.

Solution:

Let’s consider the roots of the quadratic equation to be a and b.

Then, we have

We know that,

A quadratic equation can be formed as,

x² – x (Sum of roots) + (Product of roots) = 0

x² – x (a + b) + (ab) = 0

x² – 16x + 25 = 0 [Using (1) and (2)]

Therefore, the required quadratic equation is x² – 16x + 25 = 0

Exercise 9.4 Page No: 196

Find the sum to n terms of each of the series in Exercises 1 to 7.

1. 1 × 2 + 2 × 3 + 3 × 4 + 4 × 5 + …

Solution:

Given series is 1 × 2 + 2 × 3 + 3 × 4 + 4 × 5 + …

It’s seen that,

n^th term, a_n = n ( n + 1)

Then, the sum of n terms of the series can be expressed as

2. 1 × 2 × 3 + 2 × 3 × 4 + 3 × 4 × 5 + …

Solution:

Given series is 1 × 2 × 3 + 2 × 3 × 4 + 3 × 4 × 5 + …

It’s seen that,

n^th term, a_n = n ( n + 1) ( n + 2)

= (n² + n) (n + 2)

= n³ + 3n² + 2n

Then, the sum of n terms of the series can be expressed as

3. 3 × 1² + 5 × 2² + 7 × 3² + …

Solution:

Given series is 3 ×1² + 5 × 2² + 7 × 3² + …

It’s seen that,

n^th term, a_n = ( 2n + 1) n² = 2n³ + n²

Then, the sum of n terms of the series can be expressed as

4. Find the sum to n terms of the series 

Solution:

5. Find the sum to n terms of the series 5² + 6² + 7² + … + 20²

Solution:

Given series is 5² + 6² + 7² + … + 20²

It’s seen that,

n^th term, a_n = ( n + 4)² = n² + 8n + 16

Then, the sum of n terms of the series can be expressed as

6. Find the sum to n terms of the series 3 × 8 + 6 × 11 + 9 × 14 +…

Solution:

Given series is 3 × 8 + 6 × 11 + 9 × 14 + …

It’s found out that,

a_n = (n^th term of 3, 6, 9 …) × (n^th term of 8, 11, 14, …)

= (3n) (3n + 5)

= 9n² + 15n

Then, the sum of n terms of the series can be expressed as

7. Find the sum to n terms of the series 1² + (1² + 2²) + (1² + 2² + 3²) + …

Solution:

Given series is 1² + (1² + 2²) + (1² + 2² + 3² ) + …

Finding the n^th term, we have

a_n = (1² + 2² + 3² +…….+ n²)

Now, the sum of n terms of the series can be expressed as

8. Find the sum to n terms of the series whose n^th term is given by n (n + 1) (n + 4).

Solution:

Given,

a_n = n (n + 1) (n + 4) = n(n² + 5n + 4) = n³ + 5n² + 4n

Now, the sum of n terms of the series can be expressed as

9. Find the sum to n terms of the series whose n^th term is given by n² + 2ⁿ

Solution:

Given,

The n^th term of the series as

a_n = n² + 2ⁿ

Then, the sum of n terms of the series can be expressed as

10. Find the sum to n terms of the series whose n^th term is given by (2n – 1)²

Solution:

Given,

The n^th term of the series as:

a_n = (2n – 1)² = 4n² – 4n + 1

Then, the sum of n terms of the series can be expressed as

Miscellaneous Exercise Page No: 199

1. Show that the sum of (m + n)^th and (m – n)^th terms of an A.P. is equal to twice the m^th term.

Solution:

Let’s take a and d to be the first term and the common difference of the A.P., respectively.

We know that the k^th term of an A. P. is given by

a_k = a + (k –1) d

So, a_m + n = a + (m + n –1) d

And, a_m – n = a + (m – n –1) d

a_m = a + (m –1) d

Thus,

a_m + n + a_m – n  = a + (m + n –1) d + a + (m – n –1) d

= 2a + (m + n –1 + m – n –1) d

= 2a + (2m – 2) d

= 2a + 2 (m – 1) d

=2 [a + (m – 1) d]

= 2a_m

Therefore, the sum of (m + n)^th and (m – n)^th terms of an A.P. is equal to twice the m^th term

2. If the sum of three numbers in A.P. is 24 and their product is 440, find the numbers.

Solution:

Let’s consider the three numbers in A.P. as a – d, a, and a + d.

Then, from the question we have

(a – d) + (a) + (a + d) = 24 … (i)

3a = 24

∴ a = 8

And,

(a – d) a (a + d) = 440 … (ii)

(8 – d) (8) (8 + d) = 440

(8 – d) (8 + d) = 55

64 – d² = 55

d² = 64 – 55 = 9

∴ d = ± 3

Thus,

When d = 3, the numbers are 5, 8, and 11 and

When d = –3, the numbers are 11, 8, and 5.

Therefore, the three numbers are 5, 8, and 11.

3. Let the sum of n, 2n, 3n terms of an A.P. be S₁, S₂ and S₃, respectively, show that S₃ = 3 (S₂– S₁).

Solution:

Let’s take a and d to be the first term and the common difference of the A.P., respectively.

So, we have

4. Find the sum of all numbers between 200 and 400, which are divisible by 7.

Solution:

First, let’s find the numbers between 200 and 400, which are divisible by 7.

The numbers are:

203, 210, 217, … 399

Here, the first term, a = 203

Last term, l = 399 and

Common difference, d = 7

Let’s consider the number of terms of the A.P. to be n.

Hence, a_n = 399 = a + (n –1) d

399 = 203 + (n –1) 7

7 (n –1) = 196

n –1 = 28

n = 29

Then, the sum of 29 terms of the A.P is given by:

Therefore, the required sum is 8729.

5. Find the sum of integers from 1 to 100 that are divisible by 2 or 5.

Solution:

First let’s find the integers from 1 to 100, which are divisible by 2.

And they are 2, 4, 6… 100.

Clearly, this forms an A.P. with the first term and common difference both equal to 2.

So, we have

100 = 2 + (n –1) 2

n = 50

Hence, the sum is

Now, the integers from 1 to 100, which are divisible by 5, are 5, 10… 100.

This also forms an A.P. with the first term and common difference both equal to 5.

So, we have

100 = 5 + (n –1) 5

5n = 100

n = 20

Hence, the sum is

Lastly, the integers, which are divisible by both 2 and 5, are 10, 20, … 100.

And this also forms an A.P. with the first term and common difference both equal to 10.

So, we have

100 = 10 + (n –1) (10)

100 = 10n

n = 10

Thus, the required sum = 2550 + 1050 – 550 = 3050

Therefore, the sum of the integers from 1 to 100, which are divisible by 2 or 5, is 3050.

6. Find the sum of all two-digit numbers, which, when divided by 4, yields 1 as the remainder.

Solution:

We have to first find the two-digit numbers, which, when divided by 4, yield 1 as the remainder.

They are 13, 17, … 97.

As it’s seen that this series forms an A.P. with the first term (a) 13 and common difference (d) 4.

Let n be the number of terms of the A.P.

We know that the n^th term of an A.P. is given by

a_n = a + (n –1) d

So, 97 = 13 + (n –1) (4)

4 (n –1) = 84

n – 1 = 21

n = 22

Now, the sum of n terms of an A.P. is given by,

Therefore, the required sum is 1210.

7. If f is a function satisfying f(x + y) = f(x) f(y) for all x, y ∈ N such that , find the value of n.

Solution:

Given that,

f (x + y) = f (x) × f (y) for all x, y ∈ N … (1)

f (1) = 3

Taking x = y = 1 in (1), we have

f (1 + 1) = f (2) = f (1) f (1) = 3 × 3 = 9

Similarly,

f (1 + 1 + 1) = f (3) = f (1 + 2) = f (1) f (2) = 3 × 9 = 27

And, f (4) = f (1 + 3) = f (1) f (3) = 3 × 27 = 81

Thus, f (1), f (2), f (3), …, that is 3, 9, 27, …, forms a G.P. with the first term and common ratio both equal to 3.

We know that sum of terms in G.P is given by,

And it’s given that,

Hence, the sum of the terms of the function is 120.

Therefore, the value of n is 4.

8. The sum of some terms of G.P. is 315, whose first term and the common ratio are 5 and 2, respectively. Find the last term and the number of terms.

Solution:

Given that the sum of some terms in a G.P is 315.

Let the number of terms be n.

We know that the sum of terms is

Given that the first term a is 5 and the common ratio r is 2.

Hence, the last term of the G.P = 6^th term = ar^{6 – 1} = (5)(2)⁵ = (5)(32) = 160

Therefore, the last term of the G.P. is 160.

9. The first term of a G.P. is 1. The sum of the third term and fifth term is 90. Find the common ratio of G.P.

Solution:

Let’s consider a and r to be the first term and the common ratio of the G.P., respectively.

Given, a = 1

a₃ = ar² = r²

a₅ = ar⁴ = r⁴

Then, from the question we have

r² + r⁴ = 90

r⁴ + r² – 90 = 0

Therefore, the common ratio of the G.P. is ±3.

10. The sum of three numbers in G.P. is 56. If we subtract 1, 7, and 21 from these numbers in that order, we obtain an arithmetic progression. Find the numbers.

Solution:

Let’s consider the three numbers in G.P. to be a, ar, and ar².

Then from the question, we have

a + ar + ar² = 56

a (1 + r + r²) = 56

 … (1)

Also, given

a – 1, ar – 7, ar² – 21 form an A.P.

So, (ar – 7) – (a – 1) = (ar² – 21) – (ar – 7)

ar – a – 6 = ar² – ar – 14

ar² – 2ar + a = 8

ar² – ar – ar + a = 8

a(r² + 1 – 2r) = 8

a (r – 1)² = 8 … (2)

7(r² – 2r + 1) = 1 + r + r²

7r² – 14 r + 7 – 1 – r – r² = 0

6r² – 15r + 6 = 0

6r² – 12r – 3r + 6 = 0

6r (r – 2) – 3 (r – 2) = 0

(6r – 3) (r – 2) = 0

r = 2, 1/2

When r = 2, a = 8

When r = ½, a = 32

Thus,

When r = 2, the three numbers in G.P. are 8, 16, and 32.

When r = 1/2, the three numbers in G.P. are 32, 16, and 8.

Therefore, in either case, the required three numbers are 8, 16, and 32.

11. A G.P. consists of an even number of terms. If the sum of all the terms is 5 times the sum of terms occupying odd places, then find its common ratio.

Solution:

Let’s consider the terms in the G.P.to be T₁, T₂, T₃, T₄, … T_2n.

The number of terms = 2n

Then, from the question we have

T₁ + T₂ + T₃ + …+ T_2n = 5 [T₁ + T₃ + … +T_2n–1]

T₁ + T₂ + T₃ + … + T_2n – 5 [T₁ + T₃ + … + T_2n–1] = 0

T₂ + T₄ + … + T_2n = 4 [T₁ + T₃ + … + T_2n–1] …… (1)

Now, let the terms in G.P. be a, ar, ar², ar³, …

Then (1) becomes,

[Using sum of terms in G.P.]

ar = 4a

r = 4

Thus, the common ratio of the G.P. is 4.

12. The sum of the first four terms of an A.P. is 56. The sum of the last four terms is 112. If its first term is 11, then find the number of terms.

Solution:

Let’s consider the terms in A.P. to be a, a + d, a + 2d, a + 3d, … a + (n – 2) d, a + (n – 1)d.

From the question, we have

Sum of first four terms = a + (a + d) + (a + 2d) + (a + 3d) = 4a + 6d

Sum of last four terms = [a + (n – 4) d] + [a + (n – 3) d] + [a + (n – 2) d] + [a + n – 1) d]

= 4a + (4n – 10) d

Then, according to the given condition,

4a + 6d = 56

4(11) + 6d = 56 [Since a = 11 (given)]

6d = 12

d = 2

Hence, 4a + (4n –10) d = 112

4(11) + (4n – 10)2 = 112

(4n – 10)2 = 68

4n – 10 = 34

4n = 44

n = 11

Therefore, the number of terms of the A.P. is 11.

13. If, then show that a, b, c and d are in G.P.

Solution:

Given,

On cross-multiplying, we have

Also, given

On cross-multiplying, we have

From (1) and (2), we get

b/a = c/b = d/c

Therefore, a, b, c and d are in G.P.

14. Let S be the sum, P the product and R the sum of reciprocals of n terms in a G.P. Prove that P²Rⁿ = Sⁿ

Solution:

Let the terms in G.P. be a, ar, ar², ar³, … ar^{n – 1}…

From the question, we have

Hence, P² Rⁿ = Sⁿ

15. The p^th, q^th and r^th terms of an A.P. are a, b, c, respectively.

Show that (q – r) a + (r – p) b + (p -q) c = 0

Solution:

Let’s assume t and d to be the first term and the common difference of the A.P., respectively.

Then the n^th term of the A.P. is given by a_n = t + (n – 1) d

Thus,

a_p = t + (p – 1) d = a  … (1)

a_q = t + (q – 1) d = b  … (2)

a_r = t + (r – 1) d = c  … (3)

On subtracting equation (2) from (1), we get

(p – 1 – q + 1) d = a – b

(p – q) d = a – b

On subtracting equation (3) from (2), we get

(q – 1 – r + 1) d = b – c

(q – r) d = b – c

Equating both the values of d obtained in (4) and (5), we get

Therefore, the given result is proved.

16. If aare in A.P., prove that a, b, c are in A.P.

Solution:

17. If a, b, c, d are in G.P, prove that (aⁿ + bⁿ), (bⁿ + cⁿ), (cⁿ + dⁿ) are in G.P.

Solution:

Given a, b, c, and d are in G.P.

So, we have

∴b² = ac … (i)

c² = bd … (ii)

ad = bc … (iii)

Required to prove (aⁿ + bⁿ), (bⁿ + cⁿ), (cⁿ + dⁿ) are in G.P. i.e.,

(bⁿ + cⁿ)² = (aⁿ + bⁿ) (cⁿ + dⁿ)

Taking L.H.S.

(bⁿ + cⁿ)² = b²ⁿ + 2bⁿcⁿ + c²ⁿ

= (b²)ⁿ+ 2bⁿcⁿ + (c²) ⁿ

= (ac)ⁿ + 2bⁿcⁿ + (bd)ⁿ  [Using (i) and (ii)]

= aⁿ cⁿ + bⁿcⁿ+ bⁿ cⁿ + bⁿ dⁿ

= aⁿ cⁿ + bⁿcⁿ+ aⁿ dⁿ + bⁿ dⁿ  [Using (iii)]

= cⁿ (aⁿ + bⁿ) + dⁿ (aⁿ + bⁿ)

= (aⁿ + bⁿ) (cⁿ + dⁿ)

= R.H.S.

Therefore, (aⁿ + bⁿ), (bⁿ + cⁿ), and (cⁿ + dⁿ) are in G.P

– Hence proved.

18. If a and b are the roots of x² – 3x + p = 0 and c, dare roots of x² – 12x + q = 0, where a, b, c, d, form a G.P. Prove that (q + p): (q – p) = 17:15.

Solution:

Given a and b are the roots of x² – 3x + p = 0

So, we have a + b = 3 and ab = p  … (i)

Also, c and d are the roots of x² – 12x + q = 0

So, c + d = 12 and cd = q  … (ii)

And given a, b, c, d are in G.P.

Let’s take a = x, b = xr, c = xr², d = xr³

From (i) and (ii), we get

x + xr = 3

x (1 + r) = 3

And,

xr² + xr³ =12

xr² (1 + r) = 12

On dividing, we get

When r = 2, x = 3/(1 + 2) = 3/3 = 1

When r = -2, x = 3/(1 – 2) = 3/-1 = -3

Case I:

When r = 2 and x =1,

ab = x²r = 2

cd = x²r⁵ = 32

Case II:

When r = –2, x = –3,

ab = x²r = –18

cd = x²r⁵ = – 288

Therefore, in both the cases, we get (q + p): (q – p) = 17:15\

19. The ratio of the A.M and G.M. of two positive numbers, a and b, is m: n. Show that .

Solution:

Let the two numbers be a and b.

A.M = (a + b)/ 2 and G.M. = √ab

From the question, we have

20. If a, b, c are in A.P,; b, c, d are in G.P and 1/c, 1/d, 1/e are in A.P. prove that a, c, e are in G.P.

Solution:

Given a, b, c are in A.P.

Hence, b – a = c – b  … (1)

And, given that b, c, d are in G.P.

So, c² = bd  … (2)

Also, 1/c, 1/d, 1/e are in A.P.

So,

Now, required to prove that a, c, e are in G.P. i.e., c² = ae

From (1), we have

2b = a + c

b = (a + c)/ 2

And from (2), we have

d = c²/ b

On substituting these values in (3), we get

Therefore, a, c, and e are in G.P.

21. Find the sum of the following series up to n terms:

(i) 5 + 55 + 555 + … (ii) .6 + .66 + . 666 + …

Solution:

(i) Given, 5 + 55 + 555 + …

Let S_n = 5 + 55 + 555 + ….. up to n terms

(ii) Given, .6 + .66 + . 666 + …

Let S_n = 06. + 0.66 + 0.666 + … up to n terms

22. Find the 20^th term of the series 2 × 4 + 4 × 6 + 6 × 8 + … + n terms.

Solution:

Given series is 2 × 4 + 4 × 6 + 6 × 8 + … n terms

∴ n^th term = a_n = 2n × (2n + 2) = 4n² + 4n

The 20^th term,

a₂₀ = 4 (20)² + 4(20) = 4 (400) + 80 = 1600 + 80 = 1680

Therefore, the 20^th term of the series is 1680.

23. Find the sum of the first n terms of the series: 3 + 7 + 13 + 21 + 31 + …

Solution:

The given series is 3 + 7 + 13 + 21 + 31 + …

S = 3 + 7 + 13 + 21 + 31 + …+ a_n–1 + a_n

S = 3 + 7 + 13 + 21 + …. + a_{n – 2} + a_{n – 1} + a_n

On subtracting both equations, we get

S – S = [3 + (7 + 13 + 21 + 31 + …+ a_n–1 + a_n)] – [(3 + 7 + 13 + 21 + 31 + …+ a_n–1) + a_n]

S – S = 3 + [(7 – 3) + (13 – 7) + (21 – 13) + … + (a_n – a_n–1)] – a_n

0 = 3 + [4 + 6 + 8 + … (n –1) terms] – a_n

a_n = 3 + [4 + 6 + 8 + … (n –1) terms]

24. If S₁, S₂, S₃ are the sum of the first n natural numbers, their squares and their cubes, respectively, show that 9S₂² = S₃ (1 + 8S₁).

Solution:

From the question, we have

Therefore, from (1) and (2), we have 9S₂² = S₃ (1 + 8S₁).

25. Find the sum of the following series up to n terms:

Solution:

26. Show that 

Solution:

n^th term of the numerator = n(n + 1)² = n³ + 2n² + n

n^th term of the denominator = n²(n + 1) = n³ + n²

27. A farmer buys a used tractor for Rs 12,000. He pays Rs 6,000 cash and agrees to pay the balance in annual instalments of Rs 500 plus 12% interest on the unpaid amount. How much will the tractor cost him?

Solution:

Given, the farmer pays Rs 6000 in cash.

So, the unpaid amount = Rs 12000 – Rs 6000 = Rs 6000

From the question, the interest paid annually will be

12% of 6000, 12% of 5500, 12% of 5000, …, 12% of 500

Hence, the total interest to be paid = 12% of 6000 + 12% of 5500 + 12% of 5000 + … + 12% of 500

= 12% of (6000 + 5500 + 5000 + … + 500)

= 12% of (500 + 1000 + 1500 + … + 6000)

It’s seen that the series 500, 1000, 1500 … 6000 is an A.P. with the first term and common difference both equal to 500.

Let’s take the number of terms of the A.P. to be n.

So, 6000 = 500 + (n – 1) 500

1 + (n – 1) = 12

n = 12

Now,

The sum of the A.P = 12/2 [2(500) + (12 – 1)(500)] = 6 [1000 + 5500] = 6(6500) = 39000

Hence, the total interest to be paid = 12% of (500 + 1000 + 1500 + … + 6000)

= 12% of 39000 = Rs 4680

Therefore, the tractor will cost the farmer = (Rs 12000 + Rs 4680) = Rs 16680

28. Shamshad Ali buys a scooter for Rs 22,000. He pays Rs 4,000 cash and agrees to pay the balance in annual instalments of Rs 1,000 plus 10% interest on the unpaid amount. How much will the scooter cost him?

Solution:

Given, Shamshad Ali buys a scooter for Rs 22000 and pays Rs 4000 in cash.

So, the unpaid amount = Rs 22000 – Rs 4000 = Rs 18000

From the question, it’s understood that the interest paid annually is

10% of 18000, 10% of 17000, 10% of 16000 … 10% of 1000

Hence, the total interest to be paid = 10% of 18000 + 10% of 17000 + 10% of 16000 + … + 10% of 1000

= 10% of (18000 + 17000 + 16000 + … + 1000)

= 10% of (1000 + 2000 + 3000 + … + 18000)

It’s seen that 1000, 2000, 3000 … 18000 form an A.P. with the first term and common difference both equal to 1000.

Let the number of terms be n.

So, 18000 = 1000 + (n – 1) (1000)

n = 18

Now, the sum of the A.P is given by

Thus,

Total interest paid = 10% of (18000 + 17000 + 16000 + … + 1000)

= 10% of Rs 171000 = Rs 17100

Therefore, the cost of scooter = Rs 22000 + Rs 17100 = Rs 39100

29. A person writes a letter to four of his friends. He asks each one of them to copy the letter and mail it to four different persons with the instruction that they move the chain similarly. Assuming that the chain is not broken and that it costs 50 paise to mail one letter. Find the amount spent on the postage when the 8^th set of the letter is mailed.

Solution:

It’s seen that,

The numbers of letters mailed forms a G.P.: 4, 4², … 4⁸

Here, first term = 4 and common ratio = 4

And the number of terms = 8

The sum of n terms of a G.P. is given by

Also, given that the cost to mail one letter is 50 paisa.

Hence, cost of mailing 87380 letters = Rs 87380 x (50/100) = Rs 43690 = Rs 43690

Therefore, the amount spent when the 8^th set of the letter is mailed will be Rs 43,690.

30. A man deposited Rs 10,000 in a bank at the rate of 5% simple interest annually. Find the amount in the 15^th year since he deposited the amount and also calculate the total amount after 20 years.

Solution:

Given, the man deposited Rs 10000 in a bank at the rate of 5% simple interest annually.

Hence, the interest in first year = (5/100) x Rs 10000 = Rs 500

So, the amount in the 15^th year = Rs

= Rs 10000 + 14 × Rs 500

= Rs 10000 + Rs 7000

= Rs 17000

And the amount after 20 years =

= Rs 10000 + 20 × Rs 500

= Rs 10000 + Rs 10000

= Rs 20000

Therefore, the amount in the 15^th year is Rs 17,000, and the total amount after 20 years will be Rs 20,000.

31. A manufacturer reckons that the value of a machine, which costs him Rs 15625, will depreciate each year by 20%. Find the estimated value at the end of 5 years.

Solution:

Given, the cost of the machine = Rs 15625

Also, given that the machine depreciates by 20% every year.

Hence, its value after every year is 80% of the original cost, i.e., 4/5^th of the original cost.

Therefore, the value at the end of 5 years =

= 5 × 1024 = 5120

Thus, the value of the machine at the end of 5 years will be Rs 5,120.

32. 150 workers were engaged to finish a job in a certain number of days. 4 workers dropped out on the second day, 4 more workers dropped out on the third day and so on. It took 8 more days to finish the work. Find the number of days in which the work was completed.

Solution:

Let’s assume x to be the number of days in which 150 workers finish the work.

Then, from the question, we have

150x = 150 + 146 + 142 + …. (x + 8) terms

The series 150 + 146 + 142 + …. (x + 8) terms is an A.P.

With first term (a) = 150, common difference (d) = –4 and number of terms (n) = (x + 8)

Now, finding the sum of terms

As x cannot be negative [Number of days is always a positive quantity]

x = 17

Hence, the number of days in which the work should have been completed is 17.

But, due to the dropping out of workers, the number of days in which the work is completed

= (17 + 8) = 25

NCERT Solutions for Class 11 Maths Chapter 9 – Sequences and Series

Listed below are the important concepts of Maths included in the Chapter 9 Sequences and Series:

9.1 Introduction

9.2 Sequences

9.3 Series

9.4 Arithmetic Progression (A.P.)

9.4.1 Arithmetic mean

9.5 Geometric Progression (G. P.)

9.5.1 General term of a G.P

9.5.2. Sum to n terms of a G.P

9.5.3 Geometric Mean (G.M.)

9.6 Relationship between A.M. and G.M.

9.7 Sum to n Terms of Special Series

NCERT Solutions for Class 11 Maths Chapter 9 – Sequences and Series

The chapter Sequences and Series belongs to the unit Algebra under the first term Class 11 Maths CBSE Syllabus 2023-24, which adds up to 30 marks of the total 80 marks. There are 4 exercises, along with a miscellaneous exercise, in this chapter to help students understand the concepts related to Sequences and Series clearly. Some of the topics discussed in Chapter 9 of NCERT Solutions for Class 11 Maths are as follows:

1. By a sequence, we mean an arrangement of numbers in a definite order according to some rule. Also, we define a sequence as a function whose domain is the set of natural numbers or some subsets of the type {1, 2, 3, ….k}. A sequence containing a finite number of terms is called a finite sequence. A sequence is called infinite if it is not a finite sequence.
2. Let a₁, a₂, a₃, … be the sequence, and then the sum expressed as a₁ + a₂ + a₃ + … is called a series. A series is called finite series if it has a finite number of terms.
3. An arithmetic progression (A.P.) is a sequence in which terms regularly increase or decrease by the same constant. This constant is called the common difference of the A.P. Usually, we denote the first term of A.P. by a, the common difference by d and the last term by l. The general term or the n^th term of the A.P. is given by a_n = a + (n – 1) d.
4. A sequence is said to be a geometric progression or G.P., if the ratio of any term to its preceding term is the same throughout. This constant factor is called the common ratio. Usually, we denote the first term of a G.P. by a and its common ratio by r. The general or the n^th term of G.P. is given by a_n = ar^{n – 1}

A student who has mastered the chapter Sequences and Series of Class 11 would also have a strong hold on the concepts related to the chapter, namely Sequence and Series, Arithmetic Progression (A. P.), Arithmetic Mean (A.M.), Geometric Progression (G.P.), general term of a G.P., the sum of n terms of a G.P., infinite G.P. and its sum, geometric mean (G.M.) and the relation between A.M. and G.M.

Disclaimer – 

Dropped Topics – 

9.4 Arithmetic Progression (A.P.) (up to Exercise 9.2)
9.7 Sum to n terms of Special Series
Examples 21, 22 and 24
Ques. 1–6, 12, 15, 16, 20, 23–26 (Miscellaneous Exercise)
Point 3 and 4 in the Summary