# Arithmetic Progressions Exercise 5.1

Arithmetic Progressions Exercise 5.1

Page No: 99

1. In which of the following situations, does the list of numbers involved make as arithmetic progression and why?

(i) The taxi fare after each km when the fare is Rs 15 for the first km and Rs 8 for each additional km.

It can be observed that
Taxi fare for 1st km = 15
Taxi fare for first 2 km = 15 + 8 = 23
Taxi fare for first 3 km = 23 + 8 = 31
Taxi fare for first 4 km = 31 + 8 = 39

Clearly 15, 23, 31, 39 … forms an A.P. because every term is 8 more than the preceding term.

(ii) The amount of air present in a cylinder when a vacuum pump removes 1/4 of the air remaining in the cylinder at a time.

Let the initial volume of air in a cylinder be V litres. In each stroke, the vacuum pump removes 1/4 of air remaining in the cylinder at a time. In other words, after every stroke, only 1 - 1/4 = 3/4th part of air will remain.
Therefore, volumes will be V, 3V/4 , (3V/4)2 , (3V/4)3...
Clearly, it can be observed that the adjacent terms of this series do not have the same difference between them. Therefore, this is not an A.P.

(iii) The cost of digging a well after every metre of digging, when it costs Rs 150 for the first metre and rises by Rs 50 for each subsequent metre.

Cost of digging for first metre = 150
Cost of digging for first 2 metres = 150 + 50 = 200
Cost of digging for first 3 metres = 200 + 50 = 250
Cost of digging for first 4 metres = 250 + 50 = 300
Clearly, 150, 200, 250, 300 … forms an A.P. because every term is 50 more than the preceding term.

(iv) The amount of money in the account every year, when Rs 10000 is deposited at compound interest at 8% per annum.

We know that if Rs P is deposited at r% compound interest per annum for n years, our money will be Clearly, adjacent terms of this series do not have the same difference between them. Therefore, this is not an A.P.

2. Write first four terms of the A.P. when the first term a and the common differenced are given as follows
(i) a = 10, d = 10
(ii) a = -2, d = 0
(iii) a = 4, d = - 3
(iv) a = -1 d = 1/2
(v) a = - 1.25, d = - 0.25

(i) a = 10, d = 10
Let the series be a1a2a3a4a5 …
a1 = a = 10
a2 = a1 + d = 10 + 10 = 20
a3 = a2 + d = 20 + 10 = 30
a4 = a3 + d = 30 + 10 = 40
a5 = a4 + d = 40 + 10 = 50
Therefore, the series will be 10, 20, 30, 40, 50 …
First four terms of this A.P. will be 10, 20, 30, and 40.

(ii) a = - 2, d = 0
Let the series be a1, a2a3a4 …
a1 = a = -2
a2 = a1 + d = - 2 + 0 = - 2
a3 = a2 + d = - 2 + 0 = - 2
a4 = a3 + d = - 2 + 0 = - 2
Therefore, the series will be - 2, - 2, - 2, - 2 …
First four terms of this A.P. will be - 2, - 2, - 2 and - 2.

(iii) a = 4, d = - 3
Let the series be a1a2a3a4 …
a1 = a = 4
a2 = a1 + d = 4 - 3 = 1
a3 = a2 + d = 1 - 3 = - 2
a4 = a3 + d = - 2 - 3 = - 5
Therefore, the series will be 4, 1, - 2 - 5 …
First four terms of this A.P. will be 4, 1, - 2 and - 5.

(iv) a = - 1, d = 1/2
Let the series be a1a2a3a4 …a1 = a = -1
a2 = a1 + d = -1 + 1/2 = -1/2
a3 = a2 + d = -1/2 + 1/2 = 0
a4 = a3 + d = 0 + 1/2 = 1/2
Clearly, the series will be-1, -1/2, 0, 1/2
First four terms of this A.P. will be -1, -1/2, 0 and 1/2.

(v) a = - 1.25, d = - 0.25
Let the series be a1a2a3a4 …
a1 = a = - 1.25
a2 = a1 + d = - 1.25 - 0.25 = - 1.50
a3 = a2 + d = - 1.50 - 0.25 = - 1.75
a4 = a3 + d = - 1.75 - 0.25 = - 2.00
Clearly, the series will be 1.25, - 1.50, - 1.75, - 2.00 ……..
First four terms of this A.P. will be - 1.25, - 1.50, - 1.75 and - 2.00.

3. For the following A.P.s, write the first term and the common difference.
(i) 3, 1, - 1, - 3 …
(ii) -5, - 1, 3, 7 …
(iii) 1/3, 5/3, 9/3, 13/3 ....
(iv) 0.6, 1.7, 2.8, 3.9 …

(i) 3, 1, - 1, - 3 …
Here, first term, a = 3
Common difference, d = Second term - First term
= 1 - 3 = - 2

(ii) - 5, - 1, 3, 7 …
Here, first term, a = - 5
Common difference, d = Second term - First term
= ( - 1) - ( - 5) = - 1 + 5 = 4
(iii) 1/3, 5/3, 9/3, 13/3 ....
Here, first term, a = 1/3

Common difference, d = Second term - First term

= 5/3 - 1/3 = 4/3

(iv) 0.6, 1.7, 2.8, 3.9 …
Here, first term, a = 0.6
Common difference, d = Second term - First term
= 1.7 - 0.6
= 1.1

4. Which of the following are APs? If they form an A.P. find the common difference d and write three more terms.
(i) 2, 4, 8, 16 …
(ii) 2, 5/2, 3, 7/2 ....
(iii) -1.2, -3.2, -5.2, -7.2 …
(iv) -10, - 6, - 2, 2 …
(v) 3, 3 + √2, 3 + 2√2, 3 + 3√2
(vi) 0.2, 0.22, 0.222, 0.2222 ….
(vii) 0, - 4, - 8, - 12 …
(viii) -1/2, -1/2, -1/2, -1/2 ....
(ix) 1, 3, 9, 27 …
(x) a, 2a, 3a, 4a …
(xi) aa2a3a4 …
(xii) √2, √8, √18, √32 ...
(xiii) √3, √6, √9, √12 ...
(xiv) 12, 32, 52, 72 …
(xv) 12, 52, 72, 73 …

(i) 2, 4, 8, 16 …
Here,
a2 - a1 = 4 - 2 = 2
a3 - a2 = 8 - 4 = 4
a4 - a3 = 16 - 8 = 8
⇒ an+1 - an is not the same every time.

Therefore, the given numbers are forming an A.P.

(ii) 2, 5/2, 3, 7/2 ....
Here,

a2 - a1 = 5/2 - 2 = 1/2
a3 - a2 = 3 - 5/2 = 1/2
a4 - a3 = 7/2 - 3 = 1/2
⇒ an+1 - an is same every time.
Therefore, d = 1/2 and the given numbers are in A.P.
Three more terms are
a5 = 7/2 + 1/2 = 4
a6 = 4 + 1/2 = 9/2
a7 = 9/2 + 1/2 = 5

(iii) -1.2, - 3.2, -5.2, -7.2 …
Here,
a2 - a1 = ( -3.2) - ( -1.2) = -2
a3 - a2 = ( -5.2) - ( -3.2) = -2
a4 - a3 = ( -7.2) - ( -5.2) = -2
⇒ an+1 - an is same every time.
Therefore, d = -2 and the given numbers are in A.P.
Three more terms are
a5 = - 7.2 - 2 = - 9.2
a6 = - 9.2 - 2 = - 11.2
a7 = - 11.2 - 2 = - 13.2

(iv) -10, - 6, - 2, 2 …
Here,
a2 - a1 = (-6) - (-10) = 4
a3 - a2 = (-2) - (-6) = 4
a4 - a3 = (2) - (-2) = 4
⇒ an+1 - an is same every time.
Therefore, d = 4 and the given numbers are in A.P.
Three more terms are
a5 = 2 + 4 = 6
a6 = 6 + 4 = 10
a7 = 10 + 4 = 14

(v) 3, 3 + √2, 3 + 2√2, 3 + 3√2
Here,
a2 - a1 = 3 + √2 - 3 = √2
a3 - a2 = (3 + 2√2) - (3 + √2) = √2
a4 - a3 = (3 + 3√2) - (3 + 2√2) = √2
⇒ an+1 - an is same every time.
Therefore, d = √2 and the given numbers are in A.P.
Three more terms are
a5 = (3 + √2) + √2 = 3 + 4√2
a6 = (3 + 4√2) + √2 = 3 + 5√2
a7 = (3 + 5√2) + √2 = 3 + 6√2

(vi) 0.2, 0.22, 0.222, 0.2222 ….
Here,
a2 - a1 = 0.22 - 0.2 = 0.02
a3 - a2 = 0.222 - 0.22 = 0.002
a4 - a3 = 0.2222 - 0.222 = 0.0002
⇒ an+1 - an is not the same every time.

Therefore, the given numbers are forming an A.P.

(vii) 0, -4, -8, -12 …
Here,
a2 - a1 = (-4) - 0 = -4
a3 - a2 = (-8) - (-4) = -4
a4 - a3 = (-12) - (-8) = -4
⇒ an+1 - an is same every time.
Therefore, d = -4 and the given numbers are in A.P.
Three more terms are
a5 = -12 - 4 = -16
a6 = -16 - 4 = -20
a7 = -20 - 4 = -24

(viii) -1/2, -1/2, -1/2, -1/2 ....
Here,
a2 - a1 = (-1/2) - (-1/2) = 0
a3 - a2 = (-1/2) - (-1/2) = 0
a4 - a3 = (-1/2) - (-1/2) = 0
⇒ an+1 - an is same every time.
Therefore, d = 0 and the given numbers are in A.P.
Three more terms are
a5 = (-1/2) - 0 = -1/2
a6 = (-1/2) - 0 = -1/2
a7 = (-1/2) - 0 = -1/2

(ix) 1, 3, 9, 27 …
Here,
a2 - a1 = 3 - 1 = 2
a3 - a2 = 9 - 3 = 6
a4 - a3 = 27 - 9 = 18
⇒ an+1 - an is not the same every time.

Therefore, the given numbers are forming an A.P.

(x) a, 2a, 3a, 4a …
Here,
a2 - a1 = 2a - a
a3 - a2 = 3a - 2a = a
a4 - a3 = 4a - 3a = a
⇒ an+1 - an is same every time.
Therefore, d = a and the given numbers are in A.P.
Three more terms are
a5 = 4a + a = 5a
a6 = 5a = 6a
a7 = 6a + a = 7a

(xi) aa2a3a4 …
Here,
a2 - a1 = aa = (a - 1)
a3 - a2 = a- aa(a - 1)
a4 - a3 = a4 - aa3(a - 1)
⇒ an+1 - an is not the same every time.

Therefore, the given numbers are forming an A.P.

(xii) √2, √8, √18, √32 ...
Here,
a2 - a1 = √8 - √2  = 2√2 - √2 = √2
a3 - a2 = √18 - √8 = 3√2 - 2√2  = √2
a4 - a3 = 4√2 - 3√2 = √2
⇒ an+1 - an is same every time.
Therefore, d = √2 and the given numbers are in A.P.
Three more terms are
a5 = √32  + √2 = 4√2 + √2 = 5√2 = √50
a6 = 5√2 +√2 = 6√2 = √72
a7 = 6√2 + √2 = 7√2 = √98

(xiii) √3, √6, √9, √12 ...
Here,
a2 - a1 = √6 - √3 = √3 × 2 -√3 = √3(√2 - 1)
a3 - a2 = √9 - √6 = 3 - √6 = √3(√3 - √2)
a4 - a3 = √12 - √9 = 2√3 - √3 × 3 = √3(2 - √3)
⇒ an+1 - an is not the same every time.

Therefore, the given numbers are forming an A.P.

(xiv) 12, 32, 52, 72 …

Or, 1, 9, 25, 49 …..
Here,
a2− a1 = 9 − 1 = 8
a3− a= 25 − 9 = 16
a4− a3 = 49 − 25 = 24
⇒ an+1 - an is not the same every time.

Therefore, the given numbers are forming an A.P.

(xv) 12, 52, 72, 73 …
Or 1, 25, 49, 73 …
Here,
a2− a1 = 25 − 1 = 24
a3− a= 49 − 25 = 24
a4− a3 = 73 − 49 = 24
i.e.,ak+1 − akis same every time.
⇒ an+1 - an is same every time.
Therefore, d = 24 and the given numbers are in A.P.
Three more terms are
a5= 73+ 24 = 97
a6= 97 + 24 = 121
a7= 121 + 24 = 145