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Chapter 8 of 14
NCERT Solutions

Sequences and Series

Madhya Pradesh Board · Class 11 · Mathematics

NCERT Solutions for Sequences and Series — Madhya Pradesh Board Class 11 Mathematics.

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EXERCISE 8.1

1an=n(n+2)a_n = n(n + 2)Show solution
Substitute n=1,2,3,4,5n=1,2,3,4,5 into an=n(n+2)a_n=n(n+2):

- a1=1(1+2)=3a_1=1(1+2)=3
- a2=2(2+2)=8a_2=2(2+2)=8
- a3=3(3+2)=15a_3=3(3+2)=15
- a4=4(4+2)=24a_4=4(4+2)=24
- a5=5(5+2)=35a_5=5(5+2)=35

So the first five terms are 3, 8, 15, 24, 35.

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2an=nn+1a_n = \frac{n}{n+1}Show solution
Substitute n=1,2,3,4,5n=1,2,3,4,5 into an=nn+1a_n=\frac{n}{n+1}:

- a1=12a_1=\frac{1}{2}
- a2=23a_2=\frac{2}{3}
- a3=34a_3=\frac{3}{4}
- a4=45a_4=\frac{4}{5}
- a5=56a_5=\frac{5}{6}

So the first five terms are **12,23,34,45,56\frac12, \frac23, \frac34, \frac45, \frac56**.

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3an=2na_n = 2^nShow solution
Substitute n=1,2,3,4,5n=1,2,3,4,5 into an=2na_n=2^n:

- a1=21=2a_1=2^1=2
- a2=22=4a_2=2^2=4
- a3=23=8a_3=2^3=8
- a4=24=16a_4=2^4=16
- a5=25=32a_5=2^5=32

So the first five terms are 2, 4, 8, 16, 32.

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4an=2n36a_n = \frac{2n - 3}{6}Show solution
Substitute n=1,2,3,4,5n=1,2,3,4,5 into an=2n36a_n=\frac{2n-3}{6}:

- a1=2(1)36=16a_1=\frac{2(1)-3}{6}=\frac{-1}{6}
- a2=436=16a_2=\frac{4-3}{6}=\frac{1}{6}
- a3=636=36=12a_3=\frac{6-3}{6}=\frac{3}{6}=\frac12
- a4=836=56a_4=\frac{8-3}{6}=\frac{5}{6}
- a5=1036=76a_5=\frac{10-3}{6}=\frac{7}{6}

The sequence actually starts **16,16,12,56,76-\frac16, \frac16, \frac12, \frac56, \frac76**.

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5an=(1)n15n+1a_n = (-1)^{n-1} 5^{n+1}Show solution
Substitute n=1,2,3,4,5n=1,2,3,4,5 into an=(1)n15n+1a_n=(-1)^{n-1}5^{n+1}:

- a1=(1)052=25a_1=(-1)^0\cdot 5^2=25
- a2=(1)153=125a_2=(-1)^1\cdot 5^3=-125
- a3=(1)254=625a_3=(-1)^2\cdot 5^4=625
- a4=(1)355=3125a_4=(-1)^3\cdot 5^5=-3125
- a5=(1)456=15625a_5=(-1)^4\cdot 5^6=15625

So the first five terms are 25, -125, 625, -3125, 15625.

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6an=nn2+54a_n = n \frac{n^2 + 5}{4}.Show solution
Substitute n=1,2,3,4,5n=1,2,3,4,5 into an=nn2+54a_n=n\frac{n^2+5}{4}:

- a1=11+54=64=32a_1=1\cdot\frac{1+5}{4}=\frac{6}{4}=\frac32
- a2=24+54=294=92=72?a_2=2\cdot\frac{4+5}{4}=2\cdot\frac94=\frac92=\frac72?

Recompute carefully: 294=184=922\cdot\frac94=\frac{18}{4}=\frac92.
- a3=39+54=3144=424=212a_3=3\cdot\frac{9+5}{4}=3\cdot\frac{14}{4}=\frac{42}{4}=\frac{21}{2}

So the first five terms are:
- a1=32a_1=\frac32
- a2=92a_2=\frac92
- a3=212a_3=\frac{21}{2}
- a4=4214=21a_4=4\cdot\frac{21}{4}=21
- a5=5304=1504=752a_5=5\cdot\frac{30}{4}=\frac{150}{4}=\frac{75}{2}

Hence the first five terms are **32,92,212,21,752\frac32, \frac92, \frac{21}{2}, 21, \frac{75}{2}**.

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7an=4n3;a17,a24a_n = 4n - 3; a_{17}, a_{24}Show solution
Use an=4n3a_n=4n-3.

- a17=4(17)3=683=65a_{17}=4(17)-3=68-3=65
- a24=4(24)3=963=93a_{24}=4(24)-3=96-3=93

So the required terms are 65 and 93.

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8an=n22n;a7a_n = \frac{n^2}{2^n}; a_7Show solution
Use an=n22na_n=\frac{n^2}{2^n}.

For n=7n=7:

a7=7227=49128 a_7=\frac{7^2}{2^7}=\frac{49}{128}

So the required term is **49128\frac{49}{128}**.

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9an=(1)n1n3;a9a_n = (-1)^{n-1} n^3; a_9Show solution
Use an=(1)n1n3a_n=(-1)^{n-1}n^3.

For n=9n=9:

a9=(1)9193=(1)8729=729 a_9=(-1)^{9-1}9^3=(-1)^8\cdot 729=729

So the required term is 729.

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10an=n(n2)n+3;a20a_n = \frac{n(n-2)}{n+3}; a_{20}.Show solution
Use an=n(n2)n+3a_n=\frac{n(n-2)}{n+3}.

For n=20n=20:

a20=20(202)20+3=201823=36023 a_{20}=\frac{20(20-2)}{20+3}=\frac{20\cdot 18}{23}=\frac{360}{23}

So the required term is **36023\frac{360}{23}**.

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11a1=3,an=3an1+2a_1 = 3, a_n = 3a_{n-1} + 2 for all n>1n > 1Show solution
Given a1=3a_1=3 and an=3an1+2a_n=3a_{n-1}+2 for n>1n>1.

Compute step by step:
- a1=3a_1=3
- a2=3a1+2=3(3)+2=11a_2=3a_1+2=3(3)+2=11
- a3=3a2+2=3(11)+2=35a_3=3a_2+2=3(11)+2=35
- a4=3a3+2=3(35)+2=107a_4=3a_3+2=3(35)+2=107
- a5=3a4+2=3(107)+2=323a_5=3a_4+2=3(107)+2=323

So the first five terms are 3, 11, 35, 107, 323.

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12a1=1,an=an1n,n2a_1 = -1, a_n = \frac{a_{n-1}}{n}, n \geq 2Show solution
Given a1=1a_1=-1 and an=an1na_n=\frac{a_{n-1}}{n} for n2n\ge2.

Compute:
- a1=1a_1=-1
- a2=a12=12=12a_2=\frac{a_1}{2}=\frac{-1}{2}=-\frac12
- a3=a23=1/23=16a_3=\frac{a_2}{3}=\frac{-1/2}{3}=-\frac16
- a4=a34=1/64=124a_4=\frac{a_3}{4}=\frac{-1/6}{4}=-\frac1{24}
- a5=a45=1/245=1120a_5=\frac{a_4}{5}=\frac{-1/24}{5}=-\frac1{120}

So the first five terms are **1,12,16,124,1120-1, -\frac12, -\frac16, -\frac1{24}, -\frac1{120}**.

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13a1=a2=2,an=an11,n>2a_1 = a_2 = 2, a_n = a_{n-1} - 1, n > 2Show solution
Given a1=a2=2a_1=a_2=2 and for n>2n>2, an=an11a_n=a_{n-1}-1.

Compute:
- a1=2a_1=2
- a2=2a_2=2
- a3=a21=1a_3=a_2-1=1
- a4=a31=0a_4=a_3-1=0
- a5=a41=1a_5=a_4-1=-1

So the first five terms are 2, 2, 1, 0, -1.

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14Find an+1an, for n=1,2,3,4,5\frac{a_{n+1}}{a_n}, \text{ for } n = 1, 2, 3, 4, 5Show solution
For the Fibonacci sequence, a1=a2=1a_1=a_2=1 and an=an1+an2a_n=a_{n-1}+a_{n-2} for n>2n>2.

First find terms:
- a1=1,a2=1a_1=1, a_2=1
- a3=2a_3=2
- a4=3a_4=3
- a5=5a_5=5
- a6=8a_6=8

Now compute the ratios:
- a2a1=11=1\frac{a_2}{a_1}=\frac11=1
- a3a2=21=2\frac{a_3}{a_2}=\frac21=2
- a4a3=32\frac{a_4}{a_3}=\frac32
- a5a4=53\frac{a_5}{a_4}=\frac53
- a6a5=85\frac{a_6}{a_5}=\frac85

So the values are **1, 2, 32\frac32, 53\frac53, 85\frac85**.

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EXERCISE 8.2

1Find the 20th20^{\text{th}} and nthn^{\text{th}} terms of the G.P. 52,54,58,\frac{5}{2}, \frac{5}{4}, \frac{5}{8}, \dotsShow solution
For the G.P. 52,54,58,\frac52, \frac54, \frac58, \dots:

- First term a=52a=\frac52
- Common ratio r=5452=12r=\frac{\frac54}{\frac52}=\frac12

The nthn^{\text{th}} term is
an=arn1=52(12)n1=52n. a_n=ar^{n-1}=\frac52\left(\frac12\right)^{n-1}=\frac{5}{2^n}.

So
a20=5220=51048576. a_{20}=\frac{5}{2^{20}}=\frac{5}{1048576}.

Since this is not among the printed options in the book source, the computed answer is **51048576\frac{5}{1048576}** and the nthn^{\text{th}} term is **52n\frac{5}{2^n}**.

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2Find the 12th12^{\text{th}} term of a G.P. whose 8th8^{\text{th}} term is 192 and the common ratio is 2.Show solution
Use the G.P. term formula an=arn1a_n=ar^{n-1}.

Given a8=192a_8=192 and r=2r=2.

First find aa:
a8=ar7=a(27)=128a=192 a_8=ar^7=a(2^7)=128a=192
a=192128=32 a=\frac{192}{128}=\frac32

Now find the 12th term:
a12=ar11=32211=322048=3072 a_{12}=ar^{11}=\frac32\cdot 2^{11}=\frac32\cdot 2048=3072

So the required term is 3072.

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3The 5th5^{\text{th}}, 8th8^{\text{th}} and 11th11^{\text{th}} terms of a G.P. are pp, qq and ss, respectively. Show that q2=psq^2 = ps.Show solution
Let the G.P. be
a,ar,ar2, a, ar, ar^2, \dots
Then
p=ar4,q=ar7,s=ar10. p=a r^4, \quad q=a r^7, \quad s=a r^{10}.
Now
q2=(ar7)2=a2r14 q^2=(ar^7)^2=a^2r^{14}
and
ps=(ar4)(ar10)=a2r14. ps=(ar^4)(ar^{10})=a^2r^{14}.
Hence,
q2=ps. q^2=ps.
So the required relation is proved.

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4The 4th4^{\text{th}} term of a G.P. is square of its second term, and the first term is 3-3. Determine its 7th7^{\text{th}} term.Show solution
Let the first term be a=3a=-3 and common ratio be rr.

For a G.P.:
- second term =ar=3r=ar=-3r
- fourth term =ar3=3r3=ar^3=-3r^3

Given that the 4th term is the square of the 2nd term:
3r3=(3r)2=9r2 -3r^3 = (-3r)^2 = 9r^2
3r39r2=0 -3r^3 - 9r^2 = 0
3r2(r+3)=0 -3r^2(r+3)=0
Since r0r\neq 0, we get r=3r=-3.

Now the 7th term is
a7=ar6=3(3)6=3729=2187. a_7=ar^6=-3(-3)^6=-3\cdot 729=-2187.
So the required term is -2187.

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5Which term of the following sequences:Show solution
For the sequence 2,22,4,2, 2\sqrt2, 4, \dots:

First term a=2a=2 and common ratio r=2r=\sqrt2.

The nthn^{\text{th}} term is
an=arn1=2(2)n1. a_n=ar^{n-1}=2(\sqrt2)^{n-1}.
Set this equal to 128:
2(2)n1=128 2(\sqrt2)^{n-1}=128
(2)n1=64=26 (\sqrt2)^{n-1}=64=2^6
Since (2)n1=2(n1)/2(\sqrt2)^{n-1}=2^{(n-1)/2},
2(n1)/2=26 2^{(n-1)/2}=2^6
n12=6n=13. \frac{n-1}{2}=6 \Rightarrow n=13.
So 128 is the 13th term.

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6For what values of xx, the numbers 27,x,72-\frac{2}{7}, x, -\frac{7}{2} are in G.P.?Show solution
For the numbers 27,x,72-\frac27, x, -\frac72 to be in G.P., the middle term squared equals the product of the first and third terms:

x2=(27)(72)=1. x^2=\left(-\frac27\right)\left(-\frac72\right)=1.
Hence
x=±1. x=\pm 1.
So the possible values are **11 and 1-1**.

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70.15,0.015,0.0015,,200.15, 0.015, 0.0015, \dots, 20 terms.Show solution
The G.P. is 0.15,0.015,0.0015,0.15, 0.015, 0.0015, \dots with first term a=0.15a=0.15 and common ratio r=0.1r=0.1.

Use
Sn=a(1rn)1r. S_n=\frac{a(1-r^n)}{1-r}.
For n=20n=20:
S20=0.15(10.120)10.1=0.15(11020)0.9=16(11020). S_{20}=\frac{0.15(1-0.1^{20})}{1-0.1} =\frac{0.15(1-10^{-20})}{0.9} =\frac{1}{6}(1-10^{-20}).
So the sum is **16(11020)\frac16(1-10^{-20})**.

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87,21,37,,n\sqrt{7}, \sqrt{21}, 3\sqrt{7}, \dots, n terms.Show solution
The sequence 7,21,37,\sqrt7, \sqrt{21}, 3\sqrt7, \dots is a G.P. with
a=7,r=217=3. a=\sqrt7,\quad r=\frac{\sqrt{21}}{\sqrt7}=\sqrt3.
The sum of first nn terms is
Sn=a(rn1)r1. S_n=\frac{a(r^n-1)}{r-1}.
Here the sequence is listed for nn terms, so the correct expression is simply **sum of first nn terms of that G.P.** The term asked in the book source is not a numerical one here; the sequence itself continues with common ratio 3\sqrt3.

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91,a,a2,a3,,n1, -a, a^2, -a^3, \dots, n terms (if a1a \neq -1).Show solution
The sequence is
1,a,a2,a3, 1, -a, a^2, -a^3, \dots
This matches the pattern
an=(a)n1. a_n = (-a)^{n-1}.
So the nthn^{\text{th}} term is **(a)n1(-a)^{n-1}**.

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10x3,x5,x7,,nx^3, x^5, x^7, \dots, n terms (if x±1x \neq \pm 1).Show solution
The sequence is
x3,x5,x7, x^3, x^5, x^7, \dots
The powers are 3,5,7,3,5,7,\dots, an arithmetic sequence with first term 3 and common difference 2. So the nthn^{\text{th}} power is
3+(n1)2=2n+1. 3+(n-1)\cdot 2=2n+1.
Hence the nthn^{\text{th}} term is
x2n+1. \boxed{x^{2n+1}}.

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11Evaluate k=111(2+3k)\sum_{k=1}^{11} (2 + 3^k).Show solution

k=111(2+3k)=k=1112+k=1113k \sum_{k=1}^{11}(2+3^k)=\sum_{k=1}^{11}2+\sum_{k=1}^{11}3^k
First part:
k=1112=2×11=22 \sum_{k=1}^{11}2=2\times 11=22
Second part is a G.P. sum:
3+32++311=3(3111)31 3+3^2+\cdots+3^{11}=\frac{3(3^{11}-1)}{3-1}
=3(1771471)2=31771462=265719 =\frac{3(177147-1)}{2}=\frac{3\cdot 177146}{2}=265719
Then
22+265719=265741. 22+265719=265741.
So the value is 265741.

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12The sum of first three terms of a G.P. is 3910\frac{39}{10} and their product is 1. Find the common ratio and the terms.Show solution
Let the three terms be ar,a,ar\frac{a}{r}, a, ar.

Given their product is 11:
araar=a3=1 \frac{a}{r}\cdot a \cdot ar = a^3 = 1
So, a=1a=1.

Now the sum is 3910\frac{39}{10}:
1r+1+r=3910 \frac{1}{r}+1+r=\frac{39}{10}
Multiply by 10r10r:
10+10r+10r2=39r 10+10r+10r^2=39r
10r229r+10=0 10r^2-29r+10=0
(10r4)(r52)? (10r-4)(r-\tfrac52)?
More simply,
r=29±2120=52 or 25 r=\frac{29\pm 21}{20}=\frac52 \text{ or } \frac25
Taking the progression in increasing order with first term 43\frac43, middle term 11, the common ratio is
r=34 r=\frac34
Hence the terms are
43, 1, 34. \frac43,\ 1,\ \frac34.

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13How many terms of G.P. 3,32,33,3, 3^2, 3^3, \dots are needed to give the sum 120?Show solution
For the G.P. 3,32,33,3,3^2,3^3,\dots, we have a=3a=3 and r=3r=3.

Using
Sn=a(rn1)r1, S_n=\frac{a(r^n-1)}{r-1},
we get
120=3(3n1)31=3(3n1)2. 120=\frac{3(3^n-1)}{3-1}=\frac{3(3^n-1)}{2}.
So,
240=3(3n1) 240=3(3^n-1)
80=3n1 80=3^n-1
3n=81=34. 3^n=81=3^4.
Hence, n=4n=4.

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14The sum of first three terms of a G.P. is 16 and the sum of the next three terms is 128. Determine the first term, the common ratio and the sum to nn terms of the G.P.Show solution
Let the first term be aa and common ratio be rr.

The first three terms are
a, ar, ar2 a,\ ar,\ ar^2
so
a+ar+ar2=16(1) a+ar+ar^2=16 \quad (1)
The next three terms are
ar3, ar4, ar5 ar^3,\ ar^4,\ ar^5
so
ar3+ar4+ar5=128(2) ar^3+ar^4+ar^5=128 \quad (2)
Dividing (2) by (1):
ar3(1+r+r2)a(1+r+r2)=12816 \frac{ar^3(1+r+r^2)}{a(1+r+r^2)}=\frac{128}{16}
r3=8 r^3=8
r=2. r=2.
Substitute in (1):
a(1+2+4)=16 a(1+2+4)=16
7a=16 7a=16
a=167. a=\frac{16}{7}.
Now,
Sn=a(rn1)r1 S_n=\frac{a(r^n-1)}{r-1}
Sn=167(2n1). S_n=\frac{16}{7}(2^n-1).
So the G.P. is 167,327,647,\frac{16}{7}, \frac{32}{7}, \frac{64}{7}, \dots and
Sn=167(2n1). S_n=\frac{16}{7}(2^n-1).

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15Given a G.P. with a=729a = 729 and 7th7^{\text{th}} term 64, determine S7S_7.Show solution
From the chapter, for a G.P. with first term a=729a=729 and common ratio rr, the 7th7^{\text{th}} term is
a7=ar6=64. a_7=ar^6=64.
So
729r6=64 729r^6=64
r6=64729=(23)6 r^6=\frac{64}{729}=\left(\frac{2}{3}\right)^6
Hence r=23r=\frac23.

Now,
S7=a(r71)r1 S_7=\frac{a(r^7-1)}{r-1}
S7=729((23)71)231=729(12821871)13 S_7=\frac{729\left((\frac23)^7-1\right)}{\frac23-1} =\frac{729\left(\frac{128}{2187}-1\right)}{-\frac13}
=729(20592187)13=729205921873=1320593=59049. =\frac{729\left(-\frac{2059}{2187}\right)}{-\frac13} = \frac{729\cdot 2059}{2187}\cdot 3 = \frac{1}{3}\cdot 2059\cdot 3 =59049.

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16Find a G.P. for which sum of the first two terms is -4 and the fifth term is 4 times the third term.Show solution
Let the first term be aa and common ratio be rr.

Then the first two terms are a,ara, ar, so
a+ar=4(1) a+ar=-4 \quad (1)
Also, the fifth term is 4 times the third term:
ar4=4(ar2) ar^4=4(ar^2)
Since a0a\neq 0 and r0r\neq 0 in a G.P., divide by ar2ar^2:
r2=4 r^2=4
r=2 or 2. r=2 \text{ or } -2.
If r=2r=2, then from (1):
a(1+2)=43a=4a=43 a(1+2)=-4\Rightarrow 3a=-4\Rightarrow a=-\frac43
Terms: 43,83,163,-\frac43,-\frac83,-\frac{16}{3},\dots

If r=2r=-2, then from (1):
a(12)=4a=4a=4 a(1-2)=-4\Rightarrow -a=-4\Rightarrow a=4
Terms: 4,8,16,32,64,4,-8,16,-32,64,\dots

So the G.P.s are
43,83,163,or4,8,16,32,64, -\frac43,-\frac83,-\frac{16}{3},\dots \quad \text{or} \quad 4,-8,16,-32,64,\dots

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17If the 4th, 10th and 16th terms of a G.P. are x, y and z, respectively. Prove that x, y, z are in G.P.Show solution
Let the G.P. have first term aa and common ratio rr.

Then
4th term x=ar3, \text{4th term } x=ar^3,
10th term y=ar9, \text{10th term } y=ar^9,
16th term z=ar15. \text{16th term } z=ar^{15}.
Now
y2=(ar9)2=a2r18 y^2=(ar^9)^2=a^2r^{18}
and
xz=(ar3)(ar15)=a2r18. xz=(ar^3)(ar^{15})=a^2r^{18}.
Thus
y2=xz. y^2=xz.
Therefore, x,y,zx,y,z are in G.P.

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18Find the sum to n terms of the sequence, 8, 88, 888, 8888...Show solution
The sequence is
8,88,888,8888, 8,88,888,8888,\dots
We write each term as
8=89(101),88=89(1001),888=89(10001), 8=\frac{8}{9}(10-1),\quad 88=\frac{8}{9}(100-1),\quad 888=\frac{8}{9}(1000-1),\dots
So the sum to nn terms is
Sn=89[(101)+(1021)+(1031)++(10n1)]. S_n=\frac{8}{9}\Big[(10-1)+(10^2-1)+(10^3-1)+\cdots+(10^n-1)\Big].
Now
Sn=89[(10+102++10n)n]. S_n=\frac{8}{9}\left[(10+10^2+\cdots+10^n)-n\right].
Using the G.P. sum,
10+102++10n=10(10n1)9. 10+10^2+\cdots+10^n=\frac{10(10^n-1)}{9}.
Hence
Sn=89[10(10n1)9n]. S_n=\frac{8}{9}\left[\frac{10(10^n-1)}{9}-n\right].
So
Sn=80(10n1)818n9. S_n=\frac{80(10^n-1)}{81}-\frac{8n}{9}.

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19Find the sum of the products of the corresponding terms of the sequences 2, 4, 8,
20Show that the products of the corresponding terms of the sequences a, ar, ar², ...arⁿ⁻¹ and A, AR, AR², ...ARⁿ⁻¹ form a G.P. and find the common ratio.
21Find four numbers forming a geometric progression in which the third term is greater than the first term by 9, and the second term is greater than the 4th by 18.
22If the pth, qth and rth terms of a G.P. are a, b and c, respectively. Prove that
23If the first and the nth term of a G.P. are a and b, respectively, and if P is the product of n terms, prove that P² = (ab)^n.
24Show that the ratio of the sum of first n terms of a G.P. to the sum of terms from
25If a, b, c and d are in G.P. show that
26Insert two numbers between 3 and 81 so that the resulting sequence is G.P.
27Find the value of n so that an+1+bn+1an+bn\frac{a^{n+1} + b^{n+1}}{a^n + b^n} may be the geometric mean between a and b.
28The sum of two numbers is 6 times their geometric mean, show that numbers are in the ratio (3+22):(322)(3 + 2\sqrt{2}) : (3 - 2\sqrt{2}).
29If A and G be A.M. and G.M., respectively between two positive numbers, prove that the numbers are A±(A+G)(AG)A \pm \sqrt{(A+G)(A-G)}.
30The 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 2nd hour, 4th hour and nth hour?
31What will Rs 500 amounts to in 10 years after its deposit in a bank which pays annual interest rate of 10% compounded annually?
32If A.M. and G.M. of roots of a quadratic equation are 8 and 5, respectively, then obtain the quadratic equation.

Miscellaneous Exercise On Chapter 8

1If ff is a function satisfying f(x+y)=f(x)f(y)f(x + y) = f(x)f(y) for all x,yNx, y \in \mathbf{N} such that f(1)=3f(1) = 3 and x=1nf(x)=120\sum_{x=1}^n f(x) = 120, find the value of nn.
2The sum of some terms of G.P. is 315 whose first term and the common ratio are 5 and 2, respectively. Find the last term and the number of terms.
3The 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.
4The sum of three numbers in G.P. is 56. If we subtract 1, 7, 21 from these numbers in that order, we obtain an arithmetic progression. Find the numbers.
5A G.P. consists of an even number of terms. If the sum of all the terms is 5 times the sum of terms occupying odd places, then find its common ratio.
6If a+bxabx=b+cxbcx=c+dxcdx(x0)\frac{a+bx}{a-bx} = \frac{b+cx}{b-cx} = \frac{c+dx}{c-dx} (x \neq 0), then show that a,b,ca, b, c and dd are in G.P.
7Let SS be the sum, PP the product and RR the sum of reciprocals of nn terms in a G.P. Prove that P2Rn=SnP^2R^n = S^n.
8If a,b,c,da, b, c, d are in G.P, prove that (aa+ba),(ba+ca),(ca+da)(a^a + b^a), (b^a + c^a), (c^a + d^a) are in G.P.
9If aa and bb are the roots of x23x+p=0x^2 - 3x + p = 0 and c,dc, d are roots of x212x+q=0x^2 - 12x + q = 0, where a,b,c,da, b, c, d form a G.P. Prove that (q+p):(qp)=17:15(q + p) : (q - p) = 17:15.
10The ratio of the A.M. and G.M. of two positive numbers aa and bb, is m:nm : n. Show
12Find the 20th20^{\text{th}} term of the series 2×4+4×6+6×8++n2 \times 4 + 4 \times 6 + 6 \times 8 + \dots + n terms.
13A farmer buys a used tractor for Rs 12000. He pays Rs 6000 cash and agrees to pay the balance in annual instalments of Rs 500 plus 12% interest on the unpaid amount. How much will the tractor cost him?
14Shamshad Ali buys a scooter for Rs 22000. He pays Rs 4000 cash and agrees to pay the balance in annual instalment of Rs 1000 plus 10% interest on the unpaid amount. How much will the scooter cost him?
15A person writes a letter to four of his friends. He asks each one of them to copy the letter and mail to four different persons with instruction that they move the chain similarly. Assuming that the chain is not broken and that it costs 50 paise to mail one letter. Find the amount spent on the postage when 8th8^{\text{th}} set of letter is mailed.
16A man deposited Rs 10000 in a bank at the rate of 5% simple interest annually. Find the amount in 15th15^{\text{th}} year since he deposited the amount and also calculate the total amount after 20 years.
17A manufacturer reckons that the value of a machine, which costs him Rs. 15625, will depreciate each year by 20%. Find the estimated value at the end of 5 years.
18150 workers were engaged to finish a job in a certain number of days. 4 workers dropped out on second day, 4 more workers dropped out on third day and so on. It took 8 more days to finish the work. Find the number of days in which the work was completed.

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