CLASS_11_MATHS_SOLUTIONS_NCERT

Class XI Chapter 9 – Sequences and Series Maths
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Question 19:
Find the sum of the products of the corresponding terms of the sequences 2, 4, 8, 16, 32 and 128,
32, 8, 2, 1/2.
Solution 19:
Required sum
1 1
64
4 2 1 2 2
2
Here,
First term,
1 1
4, 2,1, ,
2
2 2
a 4
Common ratio,
It is known that,
1
2128 432 88 16 2 32
2
1
r
2
S
n
is a G.P.
a 1
r
1
r
5
1
41 1
2
41
32
32 1
31
S5
8
1 1
1
32 4
2 2
n
31
64
16 31 496
4
Required sum
Question 20:
Show that the products of the corresponding terms of the sequences form
A, AR, AR , AR
2 n 1
Solution 20:
a G.P, and find the common ratio.
It has to be proved that the sequence:
Second term
First term
Third term
Second term
ar AR
rR
aA
2 2
ar AR
rR
arAR
a, ar, ar ,..... ar
2 n 1
2 2 n1 n 1
aA, arAR, ar AR , ....... ar AR
, forms a G.P.
Thus, the above sequence forms a G.P. and the common ratio is rR.
and
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