Q1. Find the sum of the following APs:
(i) 2, 7, 12, . . ., to 10 terms. (ii) –37, –33, –29, . . ., to 12 terms.
(iii) 0.6, 1.7, 2.8, . . ., to 100 terms. (iv) 1/15, 1/12, 1/10, . . ., to 11 terms.
Explanation:
1) The nth term an of an AP with the first term 'a' and common difference 'd' is given
by an = a + (n – 1) d.
2) The sum of the first n terms of an AP is given by:
S = (n/2)[2a + (n – 1) d]
3) If an = l is the last term of an AP, then
S = (n/2)[2a + (n – 1) d]
S = (n/2)[a + (a + (n – 1) d)]
S = (n/2)[a + an]
S = (n/2)[a + l].
Solution:
(i) 2, 7, 12, . . ., to 10 terms.
1) Here, a = 2, d = 7 – 2 = 5, and n = 10
2) We know that the sum of the first n terms of an AP is given by:
S = (n/2)[2a + (n – 1) d]
S = (10/2)[2(2) + (10 – 1) (5)]
S = (5)[4 + 5(9)]
S = (5)[4 + 45]
S = (5)(49)
S = 245
3) Therefore, the sum of the given AP is 245.
(ii) –37, –33, –29, . . ., to 12 terms.
1) Here, a = – 37, d = – 33 + 37 = 4, and n = 12
2) We know that the sum of the first n terms of an AP is given by:
S = (n/2)[2a + (n – 1) d]
S = (12/2)[2(– 37) + (12 – 1) (4)]
S = (6)[– 74 + 4(11)]
S = (6)[– 74 + 44]
S = (6)(– 30)
S = – 180
3) Therefore, the sum of the given AP is – 180.
(iii) 0.6, 1.7, 2.8, . . ., to 100 terms.
1) Here, a = 0.6, d = 1.7 – 0.6 = 1.1, and n = 100
2) We know that the sum of the first n terms of an AP is given by:
S = (n/2)[2a + (n – 1) d]
S = (100/2)[2(0.6) + (100 – 1) (1.1)]
S = (50)[1.2 + 1.1(99)]
S = (50)[1.2 + 108.9]
S = (50)(110.1)
S = 5505
3) Therefore, the sum of the given AP is 5505.
(iv) 1/15, 1/12, 1/10, . . ., to 11 terms.
1) Here, a = 1/15,
d = 1/12 – 1/15
d = (15 – 12)/(12x15)
d = (3)/(12x15)
d = 1/(4x15)
d = 1/60 and n = 11.
2) We know that the sum of the first n terms of an AP is given by:
S = (n/2)[2a + (n – 1) d]
S = (11/2)[2(1/15) + (11 – 1) (1/60)]
S = (11/2)[2/15 + 10/60]
S = (11/2)[2/15 + 1/6]
S = (11/2)/[(4/30) + (5/30)]
S = (11/2)/(4 + 5)/30
S = (11/2)/(9/30)
S = (11/2)/(3/10)
S = 33/20
3) Therefore, the sum of the given AP is 33/20.
Q2. Find the sums given below :
(i) 7 + 10½ + 14 + . . . + 84
(ii) 34 + 32 + 30 + . . . + 10
(iii) –5 + (–8) + (–11) + . . . + (–230)
Solution:
(i) 7 + 10½ + 14 + . . . + 84
1) Here, a = 7, d = 10½ – 7 = 3½ = 7/2, nth term is an = l = 84
2) We know that,
an = a + (n – 1) d
84 = 7 + (n – 1) (7/2)
(n – 1) (7/2) = 84 – 7
(n – 1) (7/2) = 77
(n – 1) = 77(2/7)
(n – 1) = 11(2)
(n – 1) = 22
n = 22 + 1
n = 23
3) We know that the sum of the first n terms of an AP is given by:
S = (n/2)[a + l]
S = (23/2)[7 + 84]
S = (23/2)[91]
S = 2093/2
S = 1046.5
4) Therefore, the sum of the given AP is 1046.5.
(ii) 34 + 32 + 30 + . . . + 10
1) Here, a = 34, d = 32 – 34 = – 2, nth term is an = 10
2) We know that,
an = a + (n – 1) d
10 = 34 + (n – 1) (– 2)
(n – 1) (– 2) = 10 – 34
(n – 1) (– 2) = – 24
(n – 1) = (– 24)/(– 2)
(n – 1) = 12
n = 12 + 1
n = 13
3) We know that the sum of the first n terms of an AP is given by:
S = (n/2)[a + l]
S = (13/2)[34 + 10]
S = (13/2)[44]
S = (13)[22]
S = 286
4) Therefore, the sum of the given AP is 286.
(iii) –5 + (–8) + (–11) + . . . + (–230)
1) Here, a = – 5, d = (– 8) – (– 5) = – 3, nth term is an = – 230
2) We know that,
an = a + (n – 1) d
– 230 = – 5 + (n – 1) (– 3)
(n – 1) (– 3) = 5 – 230
(n – 1) (– 3) = – 225
(n – 1) = (– 225)/(– 3)
(n – 1) = 75
n = 75 + 1
n = 76
3) We know that the sum of the first n terms of an AP is given by:
S = (n/2)[a + l]
S = (76/2)[– 5 + (– 230)]
S = (38)[– 235]
S = – 8930
4) Therefore, the sum of the given AP is – 8930.
Q3. In an AP:
(i) given a = 5, d = 3, an = 50, find n and Sn.
(ii) given a = 7, a13 = 35, find d and S13.
(iii) given a12 = 37, d = 3, find a and S12.
(iv) given a3 = 15, S10 = 125, find d and a10.
(v) given d = 5, Sn = 75, find a and a9.
(vi) given a = 2, d = 8, Sn = 90, find n and an.
(vii) given a = 8, an = 62, Sn = 210, find n and d.
(viii) given an = 4, d = 2, Sn = – 14, find n and a.
(ix) given a = 3, n = 8, S = 192, find d.
(x) given l = 28, S = 144, and there are total 9 terms. Find a.
Solution:
(i) given a = 5, d = 3, an = 50, find n and Sn.
1) Here, a = 5, d = 3, nth term is an = 50.
2) We know that,
an = a + (n – 1) d
50 = 5 + (n – 1) (3)
(n – 1) (3) = 50 – 5
(n – 1) (3) = 45
(n – 1) = 45/3
(n – 1) = 15
n = 15 + 1
n = 16
3) We know that the sum of the first n terms of an AP is given by:
S = (n/2)[a + l]
S = (16/2)[5 + 50]
S = (8)[55]
S = 440
4) Therefore, Sn = 440, n = 16.
(ii) given a = 7, a13 = 35, find d and S13.
1) Here, a = 7, d = ?, 13th term is a13 = 35. Find d and S13
2) We know that,
an = a + (n – 1) d
a13 = 7 + (13 – 1) (d)
a13 = 7 + 12d
35 = 7 + 12d
12d = 35 – 7
12d = 28
d = 28/12
d = 7/3 --------- equation 1
3) We know that the sum of the first n terms of an AP is given by:
Sn = (n/2)[a + l]
S13 = (13/2)[7 + 35]
S13 = (13/2)[42]
S13 = (13)[21]
S13 = 273
4) Therefore, S13 = 273, d = 7/3.
(iii) given a12 = 37, d = 3, find a and S12.
1) Here, a = ?, d = 3, 12th term is a12 = 37. Find "a" and S12"
2) We know that,
an = a + (n – 1) d
a12 = a + (12 – 1) (3)
37 = a + 3(11)
37 = a + 33
a = 37 – 33
a = 4 --------- equation 1
3) We know that the sum of the first n terms of an AP is given by:
Sn = (n/2)[a + l]
S12 = (12/2)[4 + 37]
S12 = (6)[41]
S12 = 246
4) Therefore, S12 = 246, a = 4.
(iv) given a3 = 15, S10 = 125, find d and a10.
1) Here, a3 = 15, S10 = 125. Find d and a10.
2) We know that,
an = a + (n – 1) d
a3 = a + (3 – 1) (d)
15 = a + 2d
a + 2d = 15 --------- equation 1
3) We know that the sum of the first n terms of an AP is given by:
Sn = (n/2)[2a + (n – 1) d]
S10 = (10/2)[2(a) + (10 – 1) d]
125 = (5)[2(a) + 9d]
[2(a) + 9d] = 25
2a + 9d = 25 --------- equation 2
4) Subtract double of equation 1 from equation 2
2a + 9d = 25
2a + 4d = 30
( – ) ( – ) ( – )
---------------------------
5d = – 5
d = – 5/5
d = – 1 --------- equation 3
5) Put d = – 1 from equation 3 in equation 1, and we get,
a + 2d = 15
a + 2(– 1) = 15
a – 2 = 15
a = 15 + 2
a = 17 --------- equation 4
6) We know that,
an = a + (n – 1) d
a10 = 17 + (10 – 1) (– 1)
a10 = 17 + 9 (– 1)
a10 = 17 – 9
a10 = 8
7) Therefore, a10 = 8, d = – 1.
(v) given d = 5, S9 = 75, find a and a9.
1) Here, d = 5, S9 = 75. Find a and a9.
2) We know that,
Sn = (n/2)[2a + (n – 1) d]
S9 = (9/2)[2a + (9 – 1) (5)]
75 = (9/2)[2a + 8 (5)]
75 = (9/2)[2a + 40]
75 = 9[a + 20]
[a + 20] = 75/9
[a + 20] = 25/3
a = (25/3) – 20
a = (25 – 60)/3
a = – 35/3
3) We know that the
an = a + (n – 1) d
a9 = – 35/3 + (9 – 1) (5)
a9 = – 35/3 + (8)(5)
a9 = – 35/3 + 40
a9 = (– 35 + 120)/3
a9 = 85/3
4) Therefore, a9 = 85/3, a = – 35/3.
(vi) given a = 2, d = 8, Sn = 90, find n and an.
1) Here, a = 2, d = 8, Sn = 90. Find n and an.
2) We know that,
Sn = (n/2)[2a + (n – 1) d]
Sn = (n/2)[2(2) + (n – 1)(8)]
90 = (n/2)[4 + 8(n – 1)]
90 = n[2 + 4(n – 1)]
90 = n(2 + 4n – 4)
90 = n(4n – 2)
90 = 4n2 – 2n
45 = 2n2 – n
2n2 – n – 45 = 0
2n2 – 10n + 9n – 45 = 0
2n(n – 5) + 9(n – 5) = 0
(n – 5)(2n + 9) = 0
(n – 5) = 0 or (2n + 9) = 0
n = 5 or 2n = – 9
n = 5 or n = – 9/2
3) As n is always a positive integer, ignore n = – 9/2. So we have n = 5.
4) We know that,
an = a + (n – 1) d
a5 = 2 + (5 – 1) (8)
a5 = 2 + (8)(4)a5 = 2 + 32
a5 = 34
5) Therefore, a5 = 34, n = 5.
(vii) given a = 8, an = 62, Sn = 210, find n and d.
1) Here, a = 8, an = 62, Sn = 210. Find n and d.
2) We know that,
Sn = (n/2)[a + l]
210 = (n/2)[8 + 62]
210 = (n/2)[70]
210 = 35n
35n = 210
n = 210/35
n = 6
3) Now we will find d using the formula:
an = a + (n – 1) d
62 = 8 + (6 – 1) (d)
62 = 8 + 5d5d = 62 – 8
5d = 54
d = 54/5
5) Therefore, d = 54/5, n = 6.
(viii) given an = 4, d = 2, Sn = – 14, find n and a.
1) Here, d = 2, an = 4, Sn = – 14. Find n and a.
2) We know that,
Sn = (n/2)[a + l]
– 14 = (n/2)[a + 4]
(n/2)[a + 4] = – 14
n[a + 4] = – 14(2)
n[a + 4] = – 28 --------- equation 1
3) We know that,
an = a + (n – 1) d
4 = a + (n – 1) (2)
4 = a + 2n – 2a + 2n = 4 + 2
a + 2n = 6
a = 6 – 2n --------- equation 2
4) Put a = 6 – 2n from equation 2 in equation 1, we get,
n(a + 4) = – 28
n((6 – 2n) + 4) = – 28
n(6 – 2n + 4) = – 28
n(10 – 2n) = – 28
n(5 – n) = – 14
n2 – 5n – 14 = 0
n(n – 7) + 2(n – 7) = 0
(n – 7)(n + 2) = 0
(n – 7) = 0 or (n + 2) = 0
n = 7 or n = – 2
5) As n is always a positive integer, ignore n = – 2. So we have n = 7.
6) Put n = 7 in equation 2, and we get,a = 6 – 2na = 6 – 2(7)
a = 6 – 14
a = – 8
7) Therefore, a = – 8, n = 7.
(ix) given a = 3, n = 8, S = 192, find d.
1) Here, a = 3, n = 8, sum of first n terms is Sn = 192, find d.
2) We know that the sum of the first n terms of an AP is given by:
S = (n/2)[2a + (n – 1) d]
192 = (8/2)[2(3) + (8 – 1) d]
192 = 4[6 + 7d]
4[6 + 7d] = 192
[6 + 7d] = 192/4
7d = 48 – 6
7d = 42
d = 42/7
d = 6
3) Therefore, d = 6.
(x) given l = 28, S = 144, and there are total 9 terms. Find a.
1) Here, l = 28, Sn = 144, n = 9, Find a.
2) We know that,
Sn = (n/2)[a + l]
144 = (9/2)[a + 28]
(9/2)[a + 28] = 144
[a + 28] = 144(2/9)
[a + 28] = 16 (2)
a + 28 = 32
a = 32 – 28
a = 4
3) Therefore, a = 4.
Q4. How many terms of the AP: 9, 17, 25, . . . must be taken to give a sum of
636?
Solution:
1) Here, a1 = a = 9, a2 = 17, a3 = 25, sum of first n terms is Sn = 636, find n.
2) According to the problem,
d = 17 – 9
d = 8
3) We know that the sum of the first n terms of an AP is given by:
S = (n/2)[2a + (n – 1) d]
636 = (n/2)[2(9) + (n – 1) (8)]
636 = (n/2)[18 + 8(n – 1)]
636 = n[9 + 4(n – 1)]
636 = n[9 + 4n – 4]
636 = 4n2 + 5n
4n2 + 5n – 636 = 0
4n2 + 53n – 48n – 636 = 0
n(4n + 53) – 12(4n + 53) = 0
(4n + 53)(n – 12) = 0
(4n + 53) = 0 or (n – 12) = 0
n = – 53/4 or n = 12
4) As n is always a positive integer, ignore n = – 53/4. So we have n = 12.
Q5. The first term of an AP is 5, the last term is 45 and the sum is 400. Find the number of terms and the common difference.
Solution:
1) Here, a1 = a = 5, an = 45, Sn = 400, find n and d.
2) We know that the sum of the first n terms of an AP is given by:
400 = (n/2)[(5) + 45]
400 = (n/2)[50]
400 = n[25]
n[25] = 400
n = 16
3) Now we will find the value of d.
45 = 5 + d (16 – 1)
15d = 45 – 5
15d = 40
d = 40/15
d = 8/3
4) Therefore, d = 8/3, n = 16.
Q6. The first and the last terms of an AP are 17 and 350 respectively. If the common difference is 9, how many terms are there, and what is their sum?
Solution:
1) Here, a1 = a = 17, an = 350, d = 9, find n and Sn.
2) Now we will find the value of d.
350 = 17 + 9(n – 1)
9(n – 1) = 350 – 17
9(n – 1) = 333
(n – 1) = 333/9
(n – 1) = 37
n = 37 + 1
n = 38
3) We know that the sum of the first n terms of an AP is given by:
Sn = (38/2)[(17) + 350]
Sn = 19[17 + 350]
Sn = 19(367)
Sn = 6973
4) Therefore, n = 38, Sn = 6973.
Q7. Find the sum of the first 22 terms of an AP in which d = 7 and the 22nd term is 149.
Solution:
1) Here, a22 = 149, d = 7, find and S22.
2) Now we will find the value of d.
149 = a + 7(22 – 1)
a = 149 – 7(21)
a = 149 – 147
a = 2
3) We know that the sum of the first n terms of an AP is given by:
S22 = (22/2)[(2) + 149]
S22 = (11)[151]
S22 = 1661
4) Therefore, a = 2, Sn = 1661.
Q8. Find the sum of the first 51 terms of an AP whose second and third terms are 14 and 18 respectively.
Solution:
1) Here, a2 = 14, a3 = 18, find and S51.
2) According to the problem,
d = 18 – 14
d = 4
3) Now we will find the value of d.
14 = a + 4(1)
a = 14 – 4
a = 10
4) We know that the sum of the first n terms of an AP is given by:
Sn = (n/2)[2a + (n – 1)d]
S51 = (51/2)[2(10) + (51 – 1)(4)]
S51 = (51/2)[20 + (50)(4)]
S51 = (51/2)[220]
S51 = 51[110]
S51 = 5610
5) Therefore, S51 = 5610.
Q9. If the sum of the first 7 terms of an AP is 49 and that of 17 terms is 289, find the sum of the first n terms.
Solution:
1) Here, S7 = 49, S17 = 289, find and Sn. Let the first term be 'a' and the common
2) We know that the sum of the first n terms of an AP is given by:
Sn = (n/2)[2a + (n – 1)d]
S7 = (7/2)[2a + (7 – 1)(d)]
49 = (7/2)[2a + (6)(d)]
49 = 7[a + 3d]
a + 3d = 49/7
a + 3d = 7 --------- equation 1
3) Now we will use S17 = 289 to get another equation
Sn = (n/2)[2a + (n – 1)d]
S17 = (17/2)[2a + (17 – 1)d]S17 = (17/2)[2a + (16)d]
S17 = 17[a + 8d]
289 = 17[a + 8d]
17[a + 8d] = 289
a + 8d = 289/17
a + 8d = 17 --------- equation 2
4) Subtract equation 1 from equation 2
a + 8d = 17
( – ) ( – ) ( – )
--------------------------
5d = 10
d = 10/5
d = 2 --------- equation 3
5) Put d = 2 from equation 3 in equation 1, and we get,
a + 3d = 7
a + 3(2) = 7
a + 6 = 7
a = 1
2) We know that the sum of the first n terms of an AP is given by:
Sn = (n/2)[2a + (n – 1)d]
Sn = (n/2)[2(1) + (n – 1)(2)]
Sn = n[n]
Sn = n2
Q10. Show that a1, a2, . . ., an, . . . form an AP where an is defined as below :
(i) an = 3 + 4n (ii) an = 9 – 5n.
Also, find the sum of the first 15 terms in each case.
Solution:
(i) an = 3 + 4n.
1) Put n = 1, 2, 3 in equation an = 3 + 4n to get a1, a2, a3 and so on.
an = 3 + 4n
a1 = 3 + 4(1)
a1 = 3 + 4a1 = 7
b) Put n = 2,
an = 3 + 4n
a2 = 3 + 4(2)
a2 = 3 + 8a2 = 11
c) Put n = 3,
an = 3 + 4n
a3 = 3 + 4(3)
a3 = 3 + 12
a3 = 15
2) Here, a1 = 7, a2 = 11, a3 = 15.
a) First difference:
d = a2 – a1
d = 4 --------- equation 1
b) Second difference:
d = a3 – a2
d = 4 --------- equation 2
3) Here the common difference d = a2 – a1 = a3 – a2 = 4,