Sequences and Series
CBSE · Class 11 · Mathematics
NCERT Solutions for Sequences and Series — CBSE Class 11 Mathematics.
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EXERCISE 8.1
1Show solution
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So the first five terms are 3, 8, 15, 24, 35.
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2Show solution
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So the first five terms are ****.
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3Show solution
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So the first five terms are 2, 4, 8, 16, 32.
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4Show solution
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The sequence actually starts ****.
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5Show solution
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So the first five terms are 25, -125, 625, -3125, 15625.
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6.Show solution
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Recompute carefully: .
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So the first five terms are:
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Hence the first five terms are ****.
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7Show solution
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So the required terms are 65 and 93.
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8Show solution
For :
So the required term is ****.
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9Show solution
For :
So the required term is 729.
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10.Show solution
For :
So the required term is ****.
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11 for all Show solution
Compute step by step:
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So the first five terms are 3, 11, 35, 107, 323.
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12Show solution
Compute:
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So the first five terms are ****.
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13Show solution
Compute:
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So the first five terms are 2, 2, 1, 0, -1.
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14Find Show solution
First find terms:
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Now compute the ratios:
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So the values are **1, 2, , , **.
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EXERCISE 8.2
1Find the and terms of the G.P. Show solution
- First term
- Common ratio
The term is
So
Since this is not among the printed options in the book source, the computed answer is **** and the term is ****.
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2Find the term of a G.P. whose term is 192 and the common ratio is 2.Show solution
Given and .
First find :
Now find the 12th term:
So the required term is 3072.
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3The , and terms of a G.P. are , and , respectively. Show that .Show solution
Then
Now
and
Hence,
So the required relation is proved.
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4The term of a G.P. is square of its second term, and the first term is . Determine its term.Show solution
For a G.P.:
- second term
- fourth term
Given that the 4th term is the square of the 2nd term:
Since , we get .
Now the 7th term is
So the required term is -2187.
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5Which term of the following sequences:Show solution
First term and common ratio .
The term is
Set this equal to 128:
Since ,
So 128 is the 13th term.
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6For what values of , the numbers are in G.P.?Show solution
Hence
So the possible values are ** and **.
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7 terms.Show solution
Use
For :
So the sum is ****.
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8 terms.Show solution
The sum of first terms is
Here the sequence is listed for terms, so the correct expression is simply **sum of first 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 .
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9 terms (if ).Show solution
This matches the pattern
So the term is ****.
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10 terms (if ).Show solution
The powers are , an arithmetic sequence with first term 3 and common difference 2. So the power is
Hence the term is
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11Evaluate .Show solution
First part:
Second part is a G.P. sum:
Then
So the value is 265741.
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12The sum of first three terms of a G.P. is and their product is 1. Find the common ratio and the terms.Show solution
Given their product is :
So, .
Now the sum is :
Multiply by :
More simply,
Taking the progression in increasing order with first term , middle term , the common ratio is
Hence the terms are
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13How many terms of G.P. are needed to give the sum 120?Show solution
Using
we get
So,
Hence, .
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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 terms of the G.P.Show solution
The first three terms are
so
The next three terms are
so
Dividing (2) by (1):
Substitute in (1):
Now,
So the G.P. is and
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15Given a G.P. with and term 64, determine .Show solution
So
Hence .
Now,
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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
Then the first two terms are , so
Also, the fifth term is 4 times the third term:
Since and in a G.P., divide by :
If , then from (1):
Terms:
If , then from (1):
Terms:
So the G.P.s are
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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
Then
Now
and
Thus
Therefore, are in G.P.
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18Find the sum to n terms of the sequence, 8, 88, 888, 8888...Show solution
We write each term as
So the sum to terms is
Now
Using the G.P. sum,
Hence
So
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Miscellaneous Exercise On Chapter 8
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Sources & Official References
- NCERT Official — ncert.nic.in
- CBSE Academic — cbseacademic.nic.in
- CBSE Official — cbse.gov.in
- National Education Policy 2020 — education.gov.in
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