NCERT Solutions
Class 10 Maths
Arithmetic Progressions

Ex. 5.1 Q.4
Which of the following are APs ? If they form an AP, 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, . . . (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) a, a2, a3, a4, . . . (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, . . .
It can be observed that a2 – a1 = 4 − 2 = 2 a3 – a2 = 8 − 4 = 4 a4 – a3 = 16 − 8 = 8
i.e., ak+1 – ak is not the same every time.
Therefore, the given numbers are not forming an A.P.
(ii) 2, 5/2, 3, 7/2, . . .
It can be observed that a2 – a1 = 5/2 − 2 = 1/2 a3 – a2 = 3 – 5/2 = 1/2 a4 – a3 = 7/2 − 3 = 1/2
i.e., ak+1 – ak is the 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
- – 1.2, – 3.2, – 5.2, – 7.2, . . . It can be observed that a2 – a1 = -3.2 – (-1.2) = -3.2 + 1.2 = -2 a3 – a2 = -5.2 – (-3.2) = -5.2 + 3.2 = -2 a4 – a3 = -7.2 – (-5.2) = -7.2 + 5.2 = -2 i.e., ak+1 – ak is the 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
- – 10, – 6, – 2, 2, . . . It can be observed that a2 – a1 = -6 – (-10) = -6 + 10 = 4 a3 – a2 = -2 – (-6) = -2 + 6 = 4 a4 – a3 = 2 – (-2) = 2 + 2 = 4
i.e., ak+1 – ak is the 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, . . .
It can be observed that a2 – a1 = 3 + √2 - 3 = √2 a3 – a2 = 3 + 2√2 - 3 - √2 = √2 a4 – a3 = 3 + 3√2 - 3 - 2√2 = √2
i.e., ak+1 – ak is the same every time.
Therefore, d = √2 and the given numbers are in A.P.
Three more terms are: a5 = 3 + 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, . . . It can be observed that a2 – a1 = 0.22 – 0.2 = 0.02 a3 – a2 = 0.222 – 0.22 = 0.002 a4 – a3 = 0.2222 – 0.222 = 0.0002
i.e., ak+1 – ak is not the same every time.
Therefore, the given numbers are not in A.P.
(vii) 0, – 4, – 8, –12, . . .
It can be observed that a2 – a1 = -4 – 0 = -4 a3 – a2 = -8 – (-4) = -8 + 4 = -4 a4 – a3 = -12 – (-8) = -12 + 8 = -4
i.e., ak+1 – ak is the 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 ….
It can be observed that a2 – a1 = (-1/2) – (-1/2) = -1/2 + 1/2 = 0 a3 – a2 = (-1/2) – (-1/2) = -1/2 + 1/2 = 0 a4 – a3 = (-1/2) – (-1/2) = -1/2 + 1/2 = 0
i.e., ak+1 – ak is the 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, . . .
It can be observed that a2 – a1 = 3 − 1 = 2 a3 – a2 = 9 − 3 = 6 a4 – a3 = 27 − 9 = 18
i.e., ak+1 – ak is not the same every time.
Therefore, the given numbers are not forming an A.P.
(x) a, 2a, 3a, 4a, . . .
It can be observed that a2 – a1 = 2a - a = a a3 – a2 = 3a - 2a = a a4 – a3 = 4a - 3a = a
i.e., ak+1 – ak is the same every time.
Therefore, d = a and the given numbers are in A.P.
Three more terms are:
a5 = 4a + a = 5a, a6 = 5a + a = 6a, a7 = 6a + a = 7a
(xi) a, a2, a3, a4, . . .
It can be observed that a2 – a1 = a2 − a = a(a - 1) a3 – a2 = a3 – a2 = a2(a - 1) a4 – a3 = a4 – a3 = a3(a – 1)
i.e., ak+1 – ak is not the same every time.
Therefore, the given numbers are not forming an A.P.
(xii) √2, √8, √18 , √32, . . . It can be observed that a2 – a1 = √8 - √2 = 2√2 - √2 = √2 a3 – a2 = √18 - √8 = 2√2 - 2√2 = √2 a4 – a3 = √32 - √18 = 4√2 - 3√2 = √2
i.e., ak+1 – ak is the 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 , . . .
It can be observed that
a2 – a1 = √6 - √2 = √(3 * 2) - √2 = √3(√2 - 1) a3 – a2 = √9 - √6 = 3 - √(3 * 2) = √3(√3 - √2) a4 – a3 = √12 - √9 = 2√3 - √(3 * 3) = √3(2 - √3)
i.e., ak+1 – ak is not the same every time.
Therefore, the given numbers are not in A.P.
(xiv) 12, 32, 52, 72, . . . = 1, 9, 25, 49, ……… It can be observed that a2 – a1 = 9 − 1 = 8 a3 – a2 = 25 − 9 = 16 a4 – a3 = 49 − 25 = 24
i.e., ak+1 – ak is not the same every time.
Therefore, the given numbers are not forming an A.P.
(xv) 12, 52, 72, 73, . . . = 1, 25, 49, 73 It can be observed that a2 – a1 = 25 − 1 = 24
a3 – a2 = 49 − 25 = 24 a4 – a3 = 73 − 49 = 24
i.e., ak+1 – ak is the same every time.
Therefore, d = 24 and the given numbers are in an A.P.
Three more terms are:
a5 = 73 + 24 = 97, a6 = 97 + 24 = 121, a7 = 121 + 24 = 145