10th Maths 5.3
Chapter 5
Arithmetic Progressions
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NCERT Class 10th solution of Exercise 5.1
NCERT Class 10th solution of Exercise 5.2
Exercise 5.3
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) 115,112,110,... to 11 terms.
Sol. :
i)
Given a=2,d=7-2=5, and n=10
Sn=n2[2a+(n-1)d]
S10=102[2(2)+(10-1)(5)]
S10=5[4+(9)(5)]
S10=5[4+45]
S10=5[49]
S10=245
Answer :
S10=245.
ii)
Given a=-37,d=(-33)-(-37)=-33+37=4, and n=12
Sn=n2[2a+(n-1)d]
S12=122[2(-37)+(12-1)(4)]
S12=6[-74+(11)(4)]
S12=6[-74+44]
S12=6[-30]
S12=-180
Answer :
S12=-180.
iii)
Given a=0.6,d=1.7-0.6=1.1, and n=100
Sn=n2[2a+(n-1)d]
S100=1002[2(0.6)+(100-1)(1.1)]
S100=50[1.2+(99)(1.1)]
S100=50[1.2+108.9]
S100=50[110.1]
S100=5505
Answer :
S100=5505.
iv)
Given a=115,d=112-115=5-460=160, and n=11
Sn=n2[2a+(n-1)d]
S11=112[2(115)+(11-1)(160)]
S11=112[215+(10)(160)]
S11=112[215+16]
S11=112[4+530]
S11=112[930]
S11=3330
Answer :
S11=3330.
Q2. Find the sums given below :
i) 7+1012+14+.....+84.
ii) 34+32+30+.....+10.
iii) -5+(-8)+(-11)+.....+(-230)
Sol. :
i)
Given a=7,d=1012-7=322=3.5, and l=an=84
an=a+(n-1)d
84=7+(n-1)(3.5)
84=7+3.5n-3.5
3.5n=84-7+3.5
n=80.53.5
n=23
Sn=n2[2a+(n-1)d]
we can write
Sn=n2[a+{a+(n-1)d}]
Sn=n2[a+{l}]
S23=232[7+84]
S23=232[91]
S23=20932
S23=104212
S23=1042.5
Answer :
S23=1046.5.
ii)
Given a=34,d=32-34=-2, and l=an=10
an=a+(n-1)d
10=34+(n-1)(-2)
10=34-2n+2
2n=36-10
n=262
n=13
Sn=n2[a+l]
S13=132[34+10]
S13=132[44]
S13=13×22
S13=286
Answer :
S13=286.
iii)
Given a=-5,d=(-8)-(-5)=-8+5=-3, and l=an=-230
an=a+(n-1)d
-230=-5+(n-1)(-3)
-230=-5+3n+3
3n=-2+230
n=2283
n=76
Sn=n2[a+l]
S76=762[-5-230]
S76=38[-235]
S76=-8930
Answer :
S76=-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,S9=75, find a and a9.
vi) given a=2,d=8,Sn=-14, 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 19 terms. Find a.
Sol. :
i)
Given a=5,d=3, and an=50
an=a+(n-1)d
50=5+(n-1)3
50=5+3n-3
3n=50-5+3
3n=48
n=483
n=16
Sn=n2(a+an)
S16=162(5+50)
S16=8(55)
S16=440
Answer :
n=16, and S16=440.
ii)
Given a=7,a13=35, and n=13
an=a+(n-1)d
35=7+(13-1)d
35=7+12d
12d=35-7
d=2812=73
Sn=n2(a+an)
S13=132(7+35)
S13=132(42)
S13=13×21
S13=273
Answer :
d=73 and S13=273
iii)
Given a12=37 and d=3
an=a+(n-1)d
37=a+(12-1)3
37=a+11×3
37=a+33
a=37-33
a=4
Sn=n2(a+an)
S12=122(4+37)
S12=6(41)
S12=246
Answer :
a=4, and S12=246.
iv)
Given a3=15,S10=125, and n=10
an=a+(n-1)d
15=a+(3-1)d
15=a+2d______________(1)
Sn=n2[2a+(n-1)d]
125=102[2a+(10-1)d]
125=5[2a+9d] [divide by 5 both sides]
25=2a+9d_____________(2)
By equation (1)×2-(2)×1
30-25=2a+4d-(2a+9d)
5=2a+4d-2a+9d
5=-5d
d=-1
Put in equation (1)
15=a+2(-1)
15=a-2
a=15+2
a=17
Now
an=a+(n-1)d
a10=17+(10-1)(-1)
a10=17-9
a10=8
Answer :
d=-1, and a10=8.
v)
Given d=5,S9=75, and n=9
Sn=n2(a+an)
75=92(a+a9)
75=3×32(a+a9)
3a+3a9=75×(23)
3a+3a9=50___________(1)
S9=92[2a+(9-1)5]
75=9a+180
9a=-180+75
a=-1059=-353
Put in equation (1)
3(-353)+3a9=50
-35+3a9=50
3a9=50+35
a9=853
Answer :
a=-353, and a9=853.
vi)
Given a=2,d=8, and Sn=90
Sn=n2[2a+(n-1)d]
90=n2[2(2)+(n-1)8]
90=n2[4+8n-8]
90=n2[8n-4]
90=4n2-2n
4n2-2n-90=0 [divide by 2 both sides]
2n2-n-45=0
2n2-10n+9n-45=0
2n(n-5)+9(n-5)=0
(n-5)(2n+9)=0
n-5=0
n=5
2n+9=0
n=-92 [ it is not possible ]
Now
an=a+(n-1)d
an=2+(5-1)8
an=2+(4)8
an=34
Answer :
n=5 and an=34.
vii)
Given a=8,an=62, and Sn=210
Sn=n2(a+an)
210=n2(8+62)
210=n2(70)
210=35n
n=21035=6
an=a+(n-1)d
62=8+(6-1)d
62=8+5d
5d=62-8
d=545
Answer :
n=6, and d=545.
viii)
Given an=4,d=2, and Sn=-14
an=a+(n-1)d
4=a+(n-1)2
4=a+2n-2
a=6-2n________________(1)
Sn=n2(a+an)
-14=n2(a+4)
-28=n(a+4)_____________(2)
from equation (1) and (2)
-28=n(6-2n+4)
-28=n(10-2n)
-28=10n-2n2
2n2-10n-28=0 [ divide by 2 both sides]
n2-5n-14=0
n2-7n+2n-14=0
n(n-7)+2(n-7)=0
(n-7)(n+2)=0
n-7=0
n=7
n+2=0
n=-2 [it is not possible ]
Put in equation (1)
a=6-2×7
a=6-14
a=-8
Answer :
a=-8, and n=7.
ix)
Given a=3,n=8, and S9=192
Sn=n2[2a+(n-1)d]
192=82[2(3)+(8-1)d]
192=4[6-7d]
48=6-7d
7d=6-48
d=427=6
Answer :
d=6.
x)
Given l=a9=28,S9=144, and n=9
S9=n2(a+l)
144=92(a+28)
288=9a+252
9a=252-288
a=369=4
Answer :
a=4.
Q4. How many terms of the AP : 9,17,25,...must be taken to give a sum of 636?
Sol. :
Given a=9,d=17-9=8, and Sn=636
Sn=n2[2a+(n-1)d]
636=n2[2(9)+(n-1)8]
636=n2[18+8n-8]
636=9n+4n2-4n
4n2+5n-636=0
4n2+53n-48n-636=0
n(4n+53)-12(4n+53)=0
(4n+53)(n-12)=0
4n+53=0
n=-534 [ it is not possible. ]
n-12=0
n=12
Answer :
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.
Sol.
Given a=5,an=45, and Sn=400
Sn=n2(a+an)
400=n2(5+45)
400=25n
n=40025
n=16
an=a(n-1)d
45=5+(16-1)d
40=15d
d=4015
d=83
Answer :
n=16, and d=83.
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?
Sol. :
Given a=17,an=350, and d=9
an=a+(n-1)d
350=17+(n-1)9
350=17+9n-9
9n=350-8
n=3429
n=38
Sn=n2(a+an)
Sn=382(17+350)
Sn=19×367
Sn=6973
Answer :
n=38, and Sn=6973.
Q7. Find the sum of first 22 terms of AP in which d=7 and 22nd term is 149.
Sol. :
Given d=7,a22=149, and n=22
an=a+(n-1)d
149=a+(22-1)7
149=a+21×7
a=149-147
a=2
Sn=n2(a+an)
S22=222(2+149)
S22=11(151)
S22=1661
Answer :
a=2, and Sn=1661.
Q8. Find the sum of first 51 terms of an AP whose second and third terms are 14 and 18 respectively.
Sol. :
Given a2=14,a3=18
a+d=14_____________(1)
a+2d=18____________(2)
By equation (2)-(1)
a+d-(a+2d)=18-14
a+d-a-2d=4
d=4
Put in equation (1)
a+4=14
a=14-4
a=10
Sn=n2[2a+(n-1)d]
S51=512[2(10)+(51-1)4]
S51=512[20+(50)4]
S51=512[20+200]
S51=512[220]
S51=51×110
S51=5610
Answer :
d=4,a=10, and S51=5610.
Q9. If the sum of first 7 terms of an AP is 49 and that of 17 terms is 289, find the sum of first n terms.
Sol. :
Given S7=49, and S17=289
Sn=n2[2a+(n-1)d]
49=72[2a+(7-1)d]
49=72[2a+6d]
497=a+3d
7=a+3d_____________(1)
S17=172[2a+(17-1)d
289=172[2a+16d]
28917=a+8d
17=a+8d____________(2)
By equation (2)-(1)
17-7=a+8d-(a+3d)
10=a+8d-a-3d
10=5d
d=105
d=2
Put in equation (1)
7=a+3×2
7=a+6
a=7-6
a=1
Sn=n2[2a+(n-1)d]
Sn=n2[2(1)+(n-1)2]
Sn=n2[2+2n-2]
Sn=n2[2n]
Sn=n2
Answer :
d=2,a=1, and 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.
Sol. :
i)
an=3+4n
a1=3+4(1)=3+4=7
a2=3+4(2)=3+8=11
a3=3+4(3)=3+12=15
....................................
an=3+4n
AP is 7,11,15,.........(3+4n)
where a=7,d=4
a1,a2,....,an,... form an AP if an=3+4n Proved.
Sn=n2[2a+(n-1)d]
S15=152[2(7)+(15-1)4]
S15=152[14+14×4]
S15=152[14+56]
S15=152[70]
S15=15×35
S15=525
Answer :
S15=525.
ii)
an=9-5n
a1=9-5(1)=9-5=4
a2=9-5(2)=9-10=-1
a3=9-5(3)=9-15=-6
................................
an=9-5n
AP is 4,-1,-6,.......(9-5n).
where a=4,d=-5
a1,a2,....,an,... form an AP if an=9-5n Proved.
Sn=n2[2a+(n-1)d]
S15=152[2(4)+(15-1)(-5)]
S15=152[8-(14)5]
Sn=152[8-70]
Sn=152[-62]
Sn=-15×31
Sn=-465
Answer :
Sn=-465.
Q11. If the sum of the first n terms of an AP is 4n-n2, what is the first term ( that is S1)? What is the sum of first two terms? what is the second term? Similarly, find the 3rd, the 10th and nth terms.
Sol. :
Given Sn=4n-n2
S1=4(1)-(1)2=4-1=3
a1=3
S2=4(2)-(2)2=8-4=4
a2=S2-S1
a2=4-3=1
d=a2-a1=1-3=-2
a3=a+2d
a3=3+2(-2)=3-6=-3
a10=a+9d
a10=3+9(-2)=3-18=-15
an=a+(n-1)d
an=3+(n-1)(-2)=3-2n+2=5-2n
Answer :
a=S1=3,S2=4,a2=1,a3=-1,a10=-15, and an=5-2n.
Q12. Find the sum of the first 40 positive integers divisible by 6.
Sol. :
AP is 6,12,18,......,40
where a=6,d=12-6=6, and n=40
Sn=n2[2a+(n-1)d]
S40=402[2(6)+(40-1)(6)]
S40=20[12+39×6]
S40=20[12+234]=20×246=4920
Answer :
S40=4920.
Q13. Find the sum of the first 15 multiples of 8.
Sol. :
AP is 8, 16, 24, .....15`
where a=8,d=16-8=8, and n=15
Sn=n2[2a+(n-1)d]
S15=152[2(8)+(15-1)8]
S15=152[16+14×8]
S15=152[16+112]
S15=152[128]
S15=15×64
S15=960
Answer :
S15=960.
Q14. Find the sum of the odd numbers between 0 and 50.
Sol. :
AP is 1,3,5,7......,47,49.
where a=1,d=3-1=2, and an=49
an=a+(n-1)d
49=1+(n-1)(2)
49=1+2n-2
49=2n-1
2n=49+1
2n=50
n=502
n=25
Sn=n2[2a+(n-1)d]
S25=252[2(1)+(25-1)2]
S25=252[2+24×2]
S25=252[2+48]
S25=252[50]
S25=25×25
S25=625
Answer :
S25=625.
Q15. A contract on construction job specifies a penalty for delay of completion beyond a certain date as follows: ₹200 for the first day, ₹250 for the second day, ₹300 for the third day etc., the penalty for each succeeding day being ₹50 more than for the preceding day. How much money the contractor has to pay as penalty, if he has delayed the worked the work by 30 days?
Sol. :
AP is 200,250,300,......a30
where a=200,d=250-200=50, and n=30
Sn=n2[2a+(n-1)d]
S30=302[2(200)+(30-1)50]
S30=15[400+29×50]
S30=15[400+1450]
S30=15[1850]
S30=27750
Answer :
Penalty is ₹ 27750.
Q16. A sum of ₹700 is to be used to give seven cash prize is ₹20 less than its preceding prize, find the value of each of the prizes.
Sol. :
Let the first prize ₹ a, and d=-₹ 20, n=7, and S7=₹ 700
Sn=n2[2a+(n-1)d]
700=72[2a+(7-1)(-20)]
700=72[2a-120]
7007=a-60
100=a-60
a=100+60
a=160
a2=a+d
a2=160-20=140
a3=a+2d
a3=160+2(-20)=160-40=120
a4=a+3d
a4=160+3(-20)=160-60=100
a5=a+4d
a5=160+4(-20)=160-80=80
a6=a+5d
a6=160+5(-20)=160-100=60
a7=a+6d
a7=160+6(-20)=160-120=40
Answer :
The prizes are ₹ 160,140,120,100,80,60, and 40.
Q17. In a school, students thought of planting trees in and around the school to reduce air pollution. It was decided that the number of trees, that each section of each class will plant, will be the same as the class, in which they are studying, e.g., a section of Class I will plant 1 tree a section of Class II will plant 2 trees and so on till Class XII. There are three sections of each class. How many trees will be planted by the students?
Sol. :
Class I will be planting 1×3=3
Class II will be planting 2×3=6
Class III will be planting 3×3=9
...................................................
Class XII will be planting 12×3=36
AP is `3, 6, 9, ....., 36.
Where a=3,d=6-3=3, and n=12
Sn=n2[2a+(n-1)d]
S12=122[2(3)+(12-1)3]
S12=6[6+11×3]
S12=6[6+33]
S12=6×39
S12=234
Answer :
234 trees will be planted by the students.
Q18. A spiral is made up of successive semicircles, with certres alternately at A and B, starting with centre at A, of radii 0.5cm, 1.0cm, 1.5cm, 2.0cm,... as shown in Figure. What is the total length of such a spiral made up of thirteen consecutive semicircles? (Take π=227)
[Hint: length of successive semicircles is l1,l2,l3,l4,.. with centres at A, B, A, B,..., respectively.]
Let length of successive semicircles is l1,l2,l3,l4,.. with centres at A, B, A, B,..., respectively.
l1=0.5πcm,
l2=1.0πcm,
l3=1.5πcm,
l4=2.0πcm,
......................
13 terms form an AP
where a=0.5πcm, d=1.0π-0.5π=0.5πcm, and n=13
Sn=n2[2a+(n-1)d]
S13=132[2(0.5π)+(13-1)0.5π]
S13=132[1.0π+12×0.5π]
S13=132[1.0π+6.0π]
S13=132[7.0π]=132[7.0×227]
S13=132[22]
S13=13×11
S13=143
Answer :
The total length of such a spiral made up of 13 consecutive semicircles is 143cm.
Q19. 200 are stacked in the following manner: 20 logs in the bottom row, 19 in the next row, 18 in the row next to it and so on (see figure). In how many rows are the 200 logs placed and how many logs are in the top row?
Sol. :
AP is 20,19,18,....,n.
where a=20,d=19-20=-1, and Sn=200
Sn=n2[2a+(n-1)d]
200=n2[2(20)+(n-1)(-1)]
200=n2[40-(n-1)]
200=n2[40-n+1]
200=n2[41-n]
400=41n-n2
n2-41n+400=0
n2-25n-16n+400=0
n(n-25)-16(n-25)=0
(n-25)(n-16)=0
n-25=0
n=25
a25=20+24(-1)=20-24=-4 [ it is not possible ]
n-16=0
n=16
a16=20+15(-1)=20-15=5
Answer :
16 rows are the 200 logs placed and 5 logs are in the top row.
Q20. In a potato race, a bucket is placed at the starting point, which is 5m from the first potato, and the other potatoes are placed 3m apart in a straight line. There are ten potatoes in the line (see figure).
A competitor starts from the bucket, picks up the nearest potato, runs back with it, drops it in the bucket, runs back to pick up the next potato, runs to the bucket to drop it in, and she continues in the same way until all the potatoes are in the bucket. What is the total distance the competitor has to run?
[ Hint : To pick up the first potato and the second potato, the total distance ( in meters) run by a competitor is 2×5+2×(5+3)]
Sol. :
a1=2×5=10m,
a2=2×(5+3)=16m,
a3=2×(8+3)=22m,
AP is 10,16,22,....
where a=10,d=16-10=6, and n=10
Sn=n2[2a+(n-1)d]
S10=102[2(10)+(10-1)6]
S10=5[20+9×6]
S10=5[20+54]
S10=5[74]
S10=370
Answer :
The total distance the competitor has to run 370m.
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