NCERT Class XI Mathematics - Binomial Theorem - Solutions

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Question : 34
Total: 36
Find n, if the ratio of the fifth term from the beginning to the fifth term from the end in the expansion of (42+
1
43
)
n
is 6 : 1.
Solution:  
Fifth term from the beginning is T4+1
i.e., T5 =
n
C4(42)n4(
1
43
)
4
=
n
C4(2)
n4
4
(3)
4
4

Fifth term from the end is Tn5+2 i.e., Tn3
Tn3 =
n
Cn4(42)4(
1
43
)
n4
=
n
C4(2)
4
4
(3)
4n
4
(Since
n
Cn4
=
n
C4
)
According to given condition, we have
n
C4(2)
n4
4
(3)1
n
C4(2)(3)
4n
4
=
6
1
2
n4
4
1
31
4n
4
= 6
2
n8
4
3
n8
4
= 23 = (2)
1
2
(3)
1
2

On comparing, we get
n8
4
=
1
2
⇒ 2n - 16 = 4 ⇒ n = 10
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