Class 9 Maths Ganita Manjari Chapter 8 Exercise 8.2 Solutions Predicting What Comes Next: Exploring Sequences and Progressions

Class 9 Maths Ganita Manjari Chapter 8 Exercise 8.2 Solutions for Predicting What Comes Next: Exploring Sequences and Progressions provides step-by-step solutions to the questions given in the exercise. Students can use these solutions to understand patterns, identify sequences and explore how terms in a progression are formed.

The detailed explanations make it easier to understand the concepts and apply the appropriate methods while solving problems. These solutions are useful for homework, revision, exam preparation and strengthening mathematical problem-solving skills.

Class 9 Maths Ganita Manjari Chapter 8 Exercise 8.2 Solutions Predicting What Comes Next: Exploring Sequences and Progressions

Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
If the sum of n terms of an A.P. is (pn + qn2), where p and q are constants, find the common difference.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Which term of the AP : 21, 18, 15, … is – 81? Also, is 0 a term of this AP? Give reasons for your answer
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
Observe the Sierpiński triangle and try to answer the following questions
(a) How many black triangles are there in Stages 0 to 3 of Fig. 8.7?
(b) Can you predict the number of black triangles at Stages 4 and 5?
(c) Can you find a rule for the number of black triangles at the nth stage? 
(d) Suppose the area of the triangle (that is, the black region) in Stage 0 is 1 square unit. What is the area of the black region in Stages 1, 2 and 3? What will be the area of the black region in Stages 4 and 5? Find a rule
for the area of the black region at the nth stage. What happens to this area as n, the number of stages, goes on increasing?
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
Can you predict the number of squares in Stages 5 and 6 of the pattern? In Stages 10, 11 and 12? In Stage 20? At any stage?

 How is this different from the growing pattern in Fig. 8.3?

Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
A child arranges marbles in rows so that the first row has 1 marble, the second has 2 marbles, the third has 3, and so on up to 25 rows. How many marbles does the child use in all?
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Harish started work at an annual salary of ₹5,00,000 and received an increment of ₹20,000 each year. After how many years did his income reach ₹7,00,000?
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
How many 2-digit numbers are divisible by 3? What is the sum of all these 2-digit numbers?
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
An AP consists of 50 terms in which the 3rd term is 12 and the last term is 106. Find the 29th term.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Find the nth term of the AP: 11, 8, 5, 2 … Write the recursive rule for this AP.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Find the 10th and 26th terms of the AP: 3, 8, 13, 18, ….
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Find the 10th and nth terms of the GP: 5, 25, 125, … .
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Let us revisit the sequence tn of triangular numbers 1, 3, 6, 10, 15, … shown in Fig. 8.1. Note that the nth term of this sequence is the sum of the first n natural numbers. thus tn=n(n+1)2\rm\;t_n=\frac{n(n+1)}{2}​.
Can you use this to find the 10th, 17th and 80th triangular numbers?
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
Can you predict the number of squares in Stages 5 and 6 of the sequence? In Stages 10, 11 and 12? In Stage 20? At any stage?
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Let T1 = 1, T2 = 2, T3 = 4, and Tn = Tn–1 + Tn–2 + Tn–3 for n ≥ 4. Find T4, T5, T6, T7, and T8.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
A sequence is given by the recursive rule t1 = – 5, tn +1=  tn + 3 for n ≥ 1. Find the first five terms of the sequence. Is 52 a term of this sequence? If so, which term is it?
Predicting What Comes Next: Exploring Sequences and Progressions
Easy
Q.
Which term of the sequence tn = 5n – 3 for n ≥ 1 is 607?
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Determine whether 97 and 172 are terms of the sequence tn = 5n – 3 for n ≥ 1.
Predicting What Comes Next: Exploring Sequences and Progressions
Easy
Q.
Find the 10th and 15th terms of the sequence tn = 5n – 3 for n ≥ 1.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Find the first five terms of the sequence in which the nth term is given by 
(i) tn = 3n – 4, 
(ii) tn = 2 – 5n, and 
(iii) tn = n2 – 2n + 3 for n ≥ 1.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Find the 12th term of a GP with common ratio 2, whose 8th term is 192.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
A sequence is given by the recursive rule  t1= 2, tn+1 = 3tn – 2 for n ≥ 1. Which term of the sequence is 730?
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
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.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
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 2nd hour, 4th hour and nth hour?
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
Suppose W1 = 1, W2 = 2 and for n > 2,  Wn = W1 + W2 + … + Wn–2 + 2.  Find the values of W1, W2, …, W8.  Do you recognise this sequence?
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
Suppose P1 = 1,  P2 = 2 and for n > 2, Pn = P1 + P2 + … + Pn–1 + 1. Find the values of P1, P2, …, P8. Can you find a simpler recursive formula for Pn ? Can you give an explicit formula?
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
The sum of the first three terms of a geometric progression is 26, and the sum of their squares is 364. Find the terms of the GP.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
If the 4th, 10th and 16th terms of a GP are x, y and z respectively, prove that x, y, z are in GP.
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
The sum of the first three terms of a GP is 1312\cfrac{13}{12}​  and their product is –1. Find the common ratio and the terms.
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
Which term of the GP: 2, 8, 32, … is 131072? Write the explicit formula as well as the recursive formula for the nth term.
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
Find the smallest value of n such that the sum of the first n natural numbers is greater than 1,000.
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
The sum of the 4th and 8th terms of an AP is 24 and the sum of the 6th and 10th terms is 44. Find the first three terms of the AP.
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
Find all possible ways of expressing 100 as the sum of consecutive natural numbers.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Which term of the GP: 2, 6, 18, … is 4374? Write the explicit formula as well as the recursive formula for the nth term.
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
Find a GP for which the sum of the first two terms is – 4 and the fifth term is 4 times the third term.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
How many multiples of 4 lie between 10 and 250?
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
How many three-digit numbers are divisible by 7?
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Determine the AP whose third term is 16 and whose 7th term exceeds the 5th term by 12.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Find the 31st term of an AP whose 11th term is 38 and 16th term is 73.
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Fig. 8.12 shows Stages 0 to 3 of the Sierpiński square carpet. Stage 0 of this fractal is a square sheet of paper. To construct Stage 1, each side of the square is trisected and the points of trisection of opposite sides are joined to obtain nine smaller squares. The centre square is then removed and the 8 smaller squares are retained, leaving a square hole in the centre. The same process is repeated on the eight smaller shaded squares to obtain Stage 2 and so on.

Look at Fig. 8.12 and try to answer the following questions.
(i) How many red squares are there in Stages 0 to 3?
(ii) Can you the number of red squares in Stages 4 and 5?
(iii) Can you find a rule for the number of red squares at the nth stage? Write the explicit formula as well as the recursive formula for the number of red squares at any stage.
(iv) Suppose the area of the square in Stage 0 is 1 square unit.
What is the area of the red region in Stages 1, 2 and 3?
What will be the area of the red region in Stages 4 and 5?
Find the explicit as well as the recursive formula for the area of the red region at the nth stage. What happens to this area as n, the number of stages, goes on increasing?
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
Which term of the sequence 2, 22,\;2\sqrt2,​4, … is 128?
Predicting What Comes Next: Exploring Sequences and Progressions
Difficult
Q.
A ball is dropped from a height of 80 metres. After hitting the ground, it bounces back to 60% of the height from which it fell. It continues bouncing in this way — each time rising to 60% of the previous height.
(i) What height does the ball reach after the 5th bounce?
(ii) What is the total vertical distance the ball has travelled by the time it hits the ground for the 6th time?
Predicting What Comes Next: Exploring Sequences and Progressions
Medium
Q.
Can you find the rule describing the nth term of the sequence of square numbers?

Class 9 Maths Ganita Manjari Chapter 8 Exercise 8.2 Solutions Predicting What Comes Next: Exploring Sequences and Progressions

NCERT Solutions for Class 9 Maths Chapter 8 Exercise Set 8.2

Exercise Set 8.2 focuses on arithmetic progressions, nth term of an AP and the sum of first n natural numbers.

Question 1. Find the 10th and 26th terms of the AP: 3, 8, 13, 18, …

Answer:
First term:

a = 3

Common difference:

d = 8 - 3 = 5

nth term of an AP:

tₙ = a + (n - 1)d

For n = 10:

t₁₀ = 3 + (10 - 1)5
= 3 + 45
= 48

For n = 26:

t₂₆ = 3 + (26 - 1)5
= 3 + 125
= 128

Final answer:
10th term = 48, 26th term = 128

Question 2. Which term of the AP 21, 18, 15, … is -81? Also, is 0 a term of this AP?

Answer:
First term:

a = 21

Common difference:

d = 18 - 21 = -3

nth term:

tₙ = 21 + (n - 1)(-3)
= 21 - 3n + 3
= 24 - 3n

For -81:

24 - 3n = -81
-3n = -105
n = 35

So, -81 is the 35th term.

Now check 0:

24 - 3n = 0
3n = 24
n = 8

Since n is a natural number, 0 is also a term.

Final answer:
-81 is the 35th term, and 0 is the 8th term.

Question 3. Find the nth term of the AP: 11, 8, 5, 2, … Write the recursive rule for this AP.

Answer:
First term:

a = 11

Common difference:

d = 8 - 11 = -3

nth term:

tₙ = a + (n - 1)d
= 11 + (n - 1)(-3)
= 11 - 3n + 3
= 14 - 3n

Recursive rule:

t₁ = 11
tₙ = tₙ₋₁ - 3, for n ≥ 2

Final answer:
tₙ = 14 - 3n; recursive rule: t₁ = 11, tₙ = tₙ₋₁ - 3

Question 4. An AP consists of 50 terms in which the 3rd term is 12 and the last term is 106. Find the 29th term.

Answer:
Let the first term be a and common difference be d.

3rd term:

a + 2d = 12 …(1)

50th term:

a + 49d = 106 …(2)

Subtract (1) from (2):

47d = 94
d = 2

Substitute in (1):

a + 2(2) = 12
a + 4 = 12
a = 8

29th term:

t₂₉ = a + 28d
= 8 + 28(2)
= 8 + 56
= 64

Final answer:
29th term = 64

Question 5. How many 2-digit numbers are divisible by 3? What is the sum of all these 2-digit numbers?

Answer:
The 2-digit numbers divisible by 3 are:

12, 15, 18, …, 99

This is an AP with:

a = 12
d = 3
last term = 99

Find n:

99 = 12 + (n - 1)3
87 = 3(n - 1)
n - 1 = 29
n = 30

So, there are 30 such numbers.

Sum:

S = n/2 × (first term + last term)

= 30/2 × (12 + 99)
= 15 × 111
= 1665

Final answer:
There are 30 two-digit numbers divisible by 3, and their sum is 1665.

Question 6. Harish started work at an annual salary of ₹5,00,000 and received an increment of ₹20,000 each year. After how many years did his income reach ₹7,00,000?

Answer:
Initial salary = ₹5,00,000
Annual increment = ₹20,000

Let the salary reach ₹7,00,000 after n years.

Increase needed:

₹7,00,000 - ₹5,00,000 = ₹2,00,000

Number of increments:

2,00,000 / 20,000 = 10

Final answer:
Harish’s income reached ₹7,00,000 after 10 years.

Question 7. A child arranges marbles in rows so that the first row has 1 marble, the second has 2 marbles, the third has 3, and so on up to 25 rows. How many marbles does the child use in all?

Answer:
Total marbles:

1 + 2 + 3 + … + 25

Sum of first n natural numbers:

Sₙ = n(n + 1)/2

For n = 25:

S₂₅ = 25(26)/2
= 25 × 13
= 325

Final answer:
325 marbles

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