NCERT Class XI Mathematics - Binomial Theorem - Solutions
© examsnet.com
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 ( 4 √ 2 +
) n is √ 6 : 1.
Solution:
Fifth term from the beginning is T 4 + 1
i.e.,T 5 =
C 4 ( 4 √ 2 ) n − 4 (
) 4 =
C 4 ( 2 )
( 3 ) −
Fifth term from the end isT n − 5 + 2 i.e., T n − 3
∴T n − 3 =
C n − 4 ( 4 √ 2 ) 4 (
) n − 4 =
C 4 ( 2 )
( 3 )
(Since
C n − 4 =
C 4 )
According to given condition, we have
=
⇒ 2
− 1 3 − 1 −
= √ 6
⇒2
3
= √ 2 √ 3 = ( 2 )
( 3 )
On comparing, we get
⇒
=
⇒ 2n - 16 = 4 ⇒ n = 10
i.e.,
Fifth term from the end is
∴
According to given condition, we have
⇒
On comparing, we get
⇒
© examsnet.com
Go to Question: